We reported previously that residents of Greater Helsinki, Finland, whose mothers were exposed to the 1957 influenza epidemic during their second trimester of gestation had a significantly elevated risk of developing adult schizophrenia. The majority of the replication studies to date have not determined whether the mothers actually contracted an infection or the stage of gestation based on mother's last menstruation. We read prenatal clinic records of the mothers of the Helsinki-born schizophrenia subjects to determine timing of infection, as noted by the prenatal clinic obstetric nurse at a time close to the actual infection. Schizophrenia subjects who were exposed in the second trimester had a significantly higher rate of definite influenza infection (86.7%) in that period compared to those who were exposed during the first and third trimesters (20.0%). These results are interpreted with caution because of the small number of cases.