Many studies indicate that both obstetrical complications (OCs) and birth in winter or early spring are risk factors for schizophrenia, but few studies have examined how these risk factors covary in the same subjects. We assessed pre- and perinatal OCs, while blind to diagnosis, using medical data recorded at the time of subjects' births, in 29 probands with DSM-III schizophrenia or schizoaffective disorder and 39 of their unaffected adult sibs. Pre- and perinatal OCs were both significantly more common in probands than sibs. Schizophrenics not born during the winter or early spring had significantly more total and perinatal OCs than schizophrenics born in other months, but did not differ for prenatal OCs. Results indicate that OCs increase risk for schizophrenia, but also suggest the possibility that the impact of OCs on this risk may be affected by season of birth.