Data on schizoaffective illness, schizophrenia and affective disorders were gathered on first-degree relatives of schizoaffective probands and matched controls (bipolars, unipolars and schizophrenics). The familial pattern of affective and schizophrenic subtypes of schizoaffective disorder resembled the familial pattern of affective and schizophrenic probands, respectively. The overall risk for the spectrum of schizoaffective and affective disorders was higher among relatives of schizoaffective-manic as compared to relatives of schizoaffective-depressive probands, although the difference fell short of significance. When tested for consistency with multiple threshold hypotheses of genetic transmission, schizoaffective illness did not qualify as either a more extreme form of affective illness nor as a disorder that occupies an intermediate position between bipolar and unipolar disorders or is genetically milder than affective disorder. The implications of diagnostic subtyping for genetic research in the major psychoses were discussed.