STUDY OBJECTIVE: To investigate the rate of suicide in the 12 months after discharge from psychiatric hospital and to determine its relationship to age, diagnosis, and period. DESIGN: Cohort study of patients discharged from psychiatric hospital. SETTING: Scotland. PARTICIPANTS: Altogether 159,742 men and 178,271 women, aged 15-84, who were discharged from Scottish psychiatric hospitals during 1968-92. MAIN RESULTS: During the 25 year period, 1212 male patients committed suicide in 198,059 person years at risk (612 per 100,000; 95% confidence interval (CI) 578,647) and 1099 female patients committed suicide in 228,993 person years at risk (480 per 100,000; 95% CI 452, 509). The overall standardised mortality ratio (general population rate = 1) was 27 (95% CI 26, 29) in men and 40 (95% CI 38, 43) in women. There were variations in the suicide rates in relation to age, diagnosis, and period. The ratio of the 1-28 day rate to the rate between days 29 and 365 over the whole study period was 1.7 (95% CI 1.4, 1.9) in men and 1.6 (95% CI 1.3, 1.8) in women. CONCLUSIONS: The variations in the post discharge suicide rate by age, sex, diagnosis, geographical location, and period suggest that there are several risk factors which vary in their distribution. Further study of these may lead to the development of effective interventions.