For a cohort of 489 people judged by a court or prosecutor in 1980 to be without responsibility for a criminal offense due to mental illness or of sufficiently diminished responsibility to be diverted for psychiatric treatment, 52 (10 percent) were arrested or convicted for further violent offenses before 1991. Violent reoffending was most strongly linked with a substance-related disorder. Histories of violence, homelessness, and short-term treatment admissions were independently related to violent reoffending. The study concludes that violent reoffending by mentally disordered offenders was so rare that it could take many years to determine any effect of the new legislation. Better indicators of the effectiveness of the new law would be cessation of substance use, compliance with treatment, and the maintenance of stable housing. These should also be the objectives of an effective preventive strategy. Of the people adjudicated in 1980 as being without responsibility or of diminished responsibility for a criminal offense due to a mental disorder, 90.4 percent (n=660) had been admitted to a hospital that year as a result of the adjudication. A total of 489 of these individuals had been discharged and were monitored from their discharge until their first violent reoffending or until December 31, 1991, whichever was sooner. These individuals composed the cohort for this study. 4 tables, 2 figures, and 35 references