OBJECTIVE: Epidemiologic studies have reported an association between prenatal exposure to influenza and adult schizophrenia. The authors studied this association in individual patients with schizophrenia and also investigated the relationship of obstetric complications, another postulated risk factor, to adult schizophrenia. METHOD: Using a structured interview instrument, the authors assessed infections during pregnancy, obstetric complications, gestational age, and birth weight by interviewing the mothers of 121 patients with DSM-III-R schizophrenia. RESULTS: Significantly more infections were reported in the second trimester of the patients' gestations than in the combined first and third trimesters. Influenza accounted for 70% of second- trimester infections. Patients with schizophrenia whose mothers reported having influenza during the second trimester were almost five times more likely to experience at least one definite obstetric complication than were patients who were not exposed to influenza during the second trimester; the exposed patients weighed a mean of 210 g less at birth than the unexposed patients. CONCLUSIONS: Maternal influenza during the second trimester may impair fetal growth and predispose to obstetric complications and lower birth weight in a proportion of individuals destined to develop schizophrenia.