A comparison of the behaviors of gang and nongang members over time showed that gang members, who composed between 9 percent and 11 percent of the sample, were significantly more involved in general antisocial behavior than nongang members both during periods of active gang involvement and during periods when they were not involved in a gang. This supports the theory of a selection effect, i.e., youth who were involved in antisocial behaviors were attracted to gangs. This effect was smaller for violence than for other types of antisocial behavior. Longitudinal analyses that compared behavioral changes in gang members and nongang members during times when gang members joined and left a gang showed that general antisocial behavior was more frequent and more violent when youth were actively involved in a gang. This finding supports the theory that being a member of a gang increases the frequency and severity of adolescent antisocial behavior. Data were collected from four relatively large cohorts of students living in Bergen, Norway, the country's second-largest city. The youth were in the fifth through the eighth grades; however, only students in grades seven and eight were asked questions about gang membership. The youth answered questions relevant to two dependent variables (general antisocial behavior and violent behavior) and the main independent variable (gang membership). Data were collected in May/June of the years 1983, 1984, and 1985. 5 tables, 2 figures, 39 references, and appended questionnaire