The age at onset of schizophrenia was determined in 100 male and 100 female patients who unequivocally met DSM-III criteria for the illness. Three different indexes of onset were used: first treatment, first hospitalization, and the immediate family's first awareness of psychotic symptoms and signs. The mean age at onset of the male patients was approximately five years earlier than that of the female patients according to all three criteria. About nine of ten male patients, compared with only two of three female patients, became schizophrenic before the age of 30 years. The onset of psychosis after the age of 35 years occurred in 17% of women and in only 2% of men. About 10% of women gave no evidence of psychosis until after the age of 40 years. The reason for the sex difference is not readily apparent, but it could be a valuable clue to some of the causes of schizophrenia.