In 1299 DSM III-R schizophrenics a slight excess of winter and spring births was evident when compared to the general population. However, when patients were allocated to different diagnostic subgroups according to the Leonhard classification this remained true only for those forms without obvious genetic loading (cycloid psychoses and systematic schizophrenias). On the contrary those forms with high genetic loading (unsystematic schizophrenias) showed a clearcut decrease of births in these months. This decrease, however, was significantly caused by periodic catatonics and cataphasics, but not by affect-laden paraphrenics. The findings corroborate the hypothesis that exogenous noxious agents, present in a crucial period of brain maturation, may be of etiological significance in schizophrenia with low genetic loading. Further, it was suggested that in some foetuses at high genetic risk for the disorder more abortions, stillbirths, postnatal deaths and early childhood deaths can occur, if additional exogenous noxious agents affect these individuals.