In a retrospective study, 16 of 80 mothers of chronic DSM III-R schizophrenics reported having had a serious infectious disease during pregnancy. Eleven of the infections had occurred during the second trimester. Influenza and the common cold with fever were frequent. Ten of 80 female controls also recalled having had an infectious illness during pregnancy. Compared to the controls, mothers of schizophrenics reported more infectious illness during pregnancy, particularly during the fifth month of gestation (p < 0.05). Mothers of familial and of sporadic DSM III-R schizophrenics reported equal frequencies of infections in pregnancy. In contrast, when Leonhard's classification of psychoses was applied, significant differences appeared. Infections during pregnancy were scarcely found in unsystematic schizophrenics (mainly genetically determined according to Leonhard). In systematic schizophrenics (mainly exogenously determined according to Leonhard), a significantly higher frequency of infectious diseases was reported for the second trimester as compred both to controls (p < 0.01) and to unsystematic schizophrenics (p < 0.001). Infections during the fifth month of gestation were exclusively reported in systematic schizophrenics. Thus, in the systematic forms of schizophrenia infections during the second trimester and particularly during the fifth month of gestation seem to play an important role in the etiology and seem to be of causal importance for the various cytoarchitectural abnormalities detected in the central nervous system of schizophrenics.