A group of schizophrenic (S) and major affective disorder (MAD) patients assessed in a Forensic Psychiatric Department was studied in depth to test the hypothesis that these patients commit crimes during periods of serious psychiatric disturbance, even if other sociodemographic factors might be contributing to their criminal behavior. The demographic, psychiatric, psychological and legal data were analyzed. The schizophrenic group was overrepresented in this sample compared with the hospitalization pattern for S and MAD patients as a whole. Both groups showed demographic characteristics such as mean age, marital and family status and education and employment, consistent with the diagnoses. S offenders tended to be either chronically ill or paranoid type and MAD offenders the early onset, bipolar type. Both groups showed evidence of serious psychiatric disturbance at the time of assessment and at the time of the alleged offense. S offenders were considered more dangerous based on clinical criteria and showed an increased tendency to assault. About two-thirds of the total group were considered unfit to stand trial at the time of assessment and about three-quarters were considered not fully responsible for the alleged offense from psychiatric viewpoint. These findings confirm the hypothesis that criminal behavior in these seriously ill patients is at least partly due to the psychiatric disturbance itself. The need for psychiatric assessment and treatment of these mentally ill offenders is thus clearly shown.