Last month, the Web site Perverted-Justice.com posted news of the conviction of Sean Young, a Wisconsin man sentenced to 10 years in state prison for soliciting sex online from a 14-year-old girl. According to a transcript of an online chat posted on the site, at one point Mr. Young had asked the girl, identified only as Billie, what she was wearing. When she answered “sweats,” Mr. Young typed back that if she were his daughter, “i’d make u wear sexy clthes.”

Billie turned out to be an adult volunteer for Perverted Justice, an anti-pedophile group, and when Mr. Young drove to a house where he expected to meet the teenager for sex, he was arrested by sheriff’s deputies.

The conviction was logged as the 104th that Perverted Justice says it has been responsible for since 2003, a tally that as of yesterday had reached 113. What started as one man’s quest to rid his regional Yahoo chat room of lewd adults has grown into a nationwide force of cyberspace vigilantes, financed by a network television program hungry for ratings.

“It’s a kind of blog that has turned into a crime-fighting resource,” said Robert McCrie, a professor at John Jay College of Criminal Justice in Manhattan.