In the region of Lower Franconia, Germany, all twins born after 1930 and hospitalized for a schizophrenia spectrum psychosis were ascertained in a systematic twin study comprising 22 monozygotic (MZ) and 25 dizygotic (DZ) pairs. One aim of the study was to compare concordance rates between MZ and DZ pairs with regard to various diagnostic classifications. Two experienced psychiatrists independently classified the probands according to DSM-III-R, ICD-10, and Leonhard's classification. Schizophrenic psychoses were found among MZ and DZ pairs in equal proportions according to DSM-III-R and ICD-10 criteria. In contrast, when Leonhard's classification was applied it became apparent that systematic schizophrenias, which represent the core group of schizophrenias in Leonhard's nosology, were completely lacking among the 34 ill MZ twins. Among the 30 ill DZ twins, 6 suffered from a systematic schizophrenia (p < 0.01). Unsystematic schizophrenias and cycloid psychoses occurred among MZ twins at a frequency of 58.8% and 41.2%, respectively. In the course of his own twin-investigations, Leonhard also observed an absence of systematic schizophrenias in MZ twins, although his twins were not systematically ascertained. This striking finding requires an explanation regarding its significance for the etiology of chronic schizophrenic psychoses. In view of the absence of other conclusive theories, one speculative explanatory model is that specific psychosocial factors, i.e., a lack of communication during childhood, may result in distinct biological damage to functional brain systems and, thus, may play a role in the pathogenesis of these psychoses.