OBJECTIVE: The authors’ goal was to determine whether there is an association between prenatal exposure to poliovirus infection and later development of schizophrenia. METHOD: All Finnish patients born between 1951 and 1969 with discharge diagnoses of schizophrenia (N=13,559) were identified from the Finnish Hospital Discharge Register. Information on the monthly number of cases of paralytic poliomyelitis was obtained for each province in Finland. The authors analyzed the incidence of births of individuals who later developed schizophrenia by using a Poisson regression model with year and place of birth, age, sex, season of birth, and smoothed incidence of poliomyelitis in different gestational periods as explanatory variables. RESULTS: An association between the incidence of poliomyelitis and the incidence of births 5 months later of individuals who later developed schizophrenia was observed. Without controlling for seasonality, the effect was significant throughout the second trimester. CONCLUSIONS: Second-trimester exposure to poliovirus infection may increase the risk for the later development of schizophrenia.