The authors studied how the abuse of alcohol and other drugs and the failure to take prescribed medications are related to behavior problems shown by young adult mentally ill persons at home and in the community. The study was based on information provided by case managers for 1,471 clients in the community support program (CSP) sponsored by the National Institute of Mental Health. The study found that alcohol abuse was strongly related to acting out among young CSP clients, but that drug abuse affected their behavior even more adversely. Failure to comply with prescribed medication regimens accounted for a considerable share of the behavior problems exhibited by the young clients, particularly acting out in the community. Age was found to have an attenuating effect on behavior problems. The authors emphasize the need to devise programs to meet the needs of young chronic mentally ill persons.