Viacom Inc.'s MTV network is on a ratings tear thanks in part to a show named "Jackass" and the network's willingness to push boundaries with stunts such as attaching blood-sucking leeches to willing victims.

This heady cocktail of gross humor and dumb jokes helped MTV's primetime ratings surge 17% in the season to date, Nielsen figures show. The edgy content isn't just pulling in MTV's biggest audiences ever. It's also attracting an older audience that previously shunned the network when it was dominated by sugarcoated boy bands like "The Backstreet Boys" and "N'Sync." MTV's audience of 25- to 34-year-olds grew 33% during the last five years, far outpacing the 17% growth in 12- to 17-year-olds during the same period.

In a coming episode of MTV's show 'Jackass,' stunt men put a beard of leeches on their faces.

But it's also becoming an increasingly risky strategy. On Tuesday, an 11-year-old Connecticut boy lit himself on fire in imitation of a stunt performed on "Jackass." Another Connecticut boy set himself on fire in January, and in February a boy in Florida did the same. The show, which is the network's top-rated feature, stars a guy who travels around with his pals performing stupid stunts, such as riding a unicycle into an open sewer, and yes, lighting himself on fire. Parents have attacked the series for provoking copycat acts among MTV's impressionable young audience, but MTV has stood by the series and reaped the results.

In an interview before the latest fire incident, Tom Freston, MTV Networks chairman and chief executive, defended the program, saying, "Jackass is nothing that MTV should be embarrassed about." He pointed out the show grew out of a subculture of skateboarders that has "its own rules and ethos." Mr. Freston added: "It's easy to dismiss it as junk. But it is a bit more thoughtful than that."

In a statement, MTV said it regrets copycat incidents: "While we don't accept responsibility, obviously we feel horrible when a young person does something to hurt themselves. Like other programmers, we take great care to air our shows responsibly."

The network runs a disclaimer on the show, which is rated TV-MA, indicating it's for adult audiences only. The rating appears on screen for 15 seconds at the start of each episode. The disclaimer reads in part: "MTV and the producers insist that neither you or anyone else attempt to recreate or perform anything you have seen." The disclaimer runs at the beginning of the half hour program and after each commercial break.

MTV was already scoring strong ratings when "Jackass" debuted in October 2000 but it has since helped lift ratings even further. The only regular show on cable TV that consistently gets a bigger rating is the World Wrestling Federation. The show attracted an average of 1.6 million viewers in the year to date, Nielsen Media Research data show. An average of 429,000 people watched MTV during the last full season, Nielsen data show, the biggest number in the network's 20-year history.

Brian Graden, MTV's programming head, says the majority of MTV's ratings growth has come from far tamer franchises, including video shows like "Total Request Live," "Making the Video" and "Say What Karaoke," plus a variety of music specials.

Still, MTV has tried pushing the envelope beyond "Jackass." Two weeks ago, a pair of 14-year old girls sued the network, claiming they were defecated on during the taping of an MTV show called "Dude, This Sucks." But they were unprepared for the featured act that calls itself "The Shower Rangers." The girls claim they were standing at the foot of the stage when the performers dropped flaps in back of their pants, exposing themselves and sprayed fecal matter on the audience.

After that incident, which the network does not dispute, Mr. Graden issued a statement saying MTV was "sorry if these women were hurt" and that he was "not aware of the content of this segment prior to the taping." The incident never aired, and MTV has since taken steps "to ensure that an incident of this nature never happens again," he added. Mr. Graden, who is widely credited with helping engineer a ratings turnaround at MTV that began four years ago, was an early executive producer of the Comedy Central hit "South Park," a show about a group of potty-mouthed grade-schoolers. "South Park" first began airing in 1997.

Other programs now on the MTV schedule include "Undressed," a soap opera about high school and college students, that delivers a nightly dose of teen sex. One recent episode featured a stepbrother and stepsister who have a sexual relationship they keep secret from their parents. Once a year, MTV's runs its "Spring Break" specials, featuring close-ups of nubile young women gyrating on stage in thin string bikinis. New video shows, such as "Total Request Live," and "Cribs," which look at rock star homes, have also been major ratings drivers.

MTV's program gambits come as other cable TV networks push the boundaries of acceptability, with shows like Comedy Central's "That's My Bush!," a parody of life in the White House that debuted April 4. One of the first episodes featured a 12-inch tall aborted fetus that talks and smokes a cigar. Comedy Central is jointly owned by Viacom and AOL Time Warner Inc.

There's also growing pressure from an explosion of new reality shows. General Electric Co.'s NBC television network is developing a new show called "Fear Factor" that will feature people doing stupid stunts. MTV already has a top-rated show called "Fear" that shows young adults going into scary places and daring each other to do stupid things like wedging themselves into collapsed tunnels. Still, some argue it's not necessary to go to the extremes of "Jackass" in order to pull in young audiences. AOL Time Warner 's WB network has a hit show with "Gilmore Girls," about a single mother, now 32, who got pregnant when she was 16. The show looks at her relationship with her daughter, who is now 16 herself, and the mom's long-estranged parents. In the season to date, an average of about 3.6 million people tuned in to watch "Gilmore Girls" -- more than double the audience for "Jackass."

Media buyers on Madison Avenue say some advertisers have requested to be taken out of the show and other edgy MTV programs. "A lot of clients that we have here are concerned about the content," says Tom Dempsey, a media buyer at closely held BCom3 Group's MediaVest media buying arm. MTV says it works with advertisers to allow them to run ads in other programs on its schedule. So far, the controversial content doesn't appear to be hurting ad revenue. Ad sales at MTV rose to more than 20% last year, data from Competitive Media Reporting shows.

Seventeen-year-old Mike Hammond says Jackass is his favorite show. "It gives the feeling that it's unedited and that it's not some corporate animal," he says. His favorite episode involves a stunt where guys put a baby carriage with what looks like a real baby on top of a car and drive around, with people screaming behind them. Mr. Hammond adds that he's never had the urge to light himself on fire after watching the show. "You'd be a real moron to do that."

Write to Sally Beatty at sally.beatty@wsj.com1