To assess the prevalence of infections in psychiatric institutes in Belgium, a cross-sectional study was performed within a cluster sample of psychiatric patients from 29 psychiatric institutes (about 40% of all Belgian psychiatric institutes). During May 1992, 8679 patients were analysed. Of the study population 59% were men. The mean age +/- SD was 54.5 +/- 18.3 years, with a median of 55 years. The mean hospitalization period was 9.6 +/- 14.5 years with a median of 2 years. A total of 1334 infections were reported, and 13.1% of the patients had a least one infection (95% CI: 12.4-13.8%) at the time of the study. The most common infections were dermatological infections (31.6%), lower respiratory tract infections (25.8%), urinary tract infections (12.4%) and conjunctivitis (11.6%). Statistically significant associations with infection prevalence were found with age, psychiatric diagnosis, activity of daily living score, the duration of hospitalization, and the qualification of the person answering the survey form. The relevance and the limitations of this study are discussed.