OBJECTIVE: Gang members engage in many high-risk activities associated with psychiatric morbidity, particularly violence-related ones. The authors investigated associations between gang membership, violent behavior, psychiatric morbidity, and use of mental health services. METHOD: The authors conducted a cross-sectional survey of 4,664 men 18-34 years of age in Great Britain using random location sampling. The survey oversampled men from areas with high levels of violence and gang activities. Participants completed questionnaires covering gang membership, violence, use of mental health services, and psychiatric diagnoses measured using standardized screening instruments. RESULTS: Violent men and gang members had higher prevalences of mental disorders and use of psychiatric services than nonviolent men, but a lower prevalence of depression. Violent ruminative thinking, violent victimization, and fear of further victimization accounted for the high levels of psychosis and anxiety disorders in gang members, and with service use in gang members and other violent men. Associations with antisocial personality disorder, substance misuse, and suicide attempts were explained by factors other than violence. CONCLUSIONS: Gang members show inordinately high levels of psychiatric morbidity, placing a heavy burden on mental health services. Traumatization and fear of further violence, exceptionally prevalent in gang members, are associated with service use. Gang membership should be routinely assessed in individuals presenting to health care services in areas with high levels of violence and gang activity. Health care professionals may have an important role in promoting desistence from gang activity.