The birthrates of 507 schizophrenic patients with high familial loading were compared to the general population. The patient group showed a clearcut decrease in births within a defined winter and spring period. In particular, this was obvious if Leonhard's classification of schizophrenia was applied. The hypothesis is proposed that, in some fetuses at high genetic risk for schizophrenia, an exaggerated level of abortions or stillbirths occurs if additional environmental noxious agents are present in a crucial period of brain maturation. Pertinent evidence in support of this is found in literature.