The study uses the ‘affected (and hospitalized) siblings method’ to investigate aspects of correlation in the psychopathology of bipolar mood disorders. In part one, age of onset, duration of phases, and course of illness are the points of interest. The sample includes 307 sibling groups (701 hospitalized individuals). The incidence of pure or schizoaffective type mania in at least 1 sibling has been the criterion for selection. The genetic background is also presented and the question is raised whether double tainting is the rule rather than the exception. The hypothesis that schizophrenic syndromes and mood disorders develop from separate genetic pools is supported. Strong evidence against the unipolar/bipolar dichotomy is presented. The investigation brings the breadth of the cyclothymic spectrum and inhomogeneity with respect to the course of bipolar disorders into perspective.