In view of tackling the problem of heterogeneity among the schizo-affectives, methods of univariate and multivariate statistical analysis (canonical discriminant analysis) were applied to the sociodemographic and natural history variables of four groups of affective disorder patients from the NIMH Collaborative Study on the Psychobiology of Depression Clinical section: the schizo-bipolar (SBP, n = 45), the schizo-unipolar (SUP, n = 30), the bipolar I (BP, n = 159) and the primary unipolar depressed (UP, n = 387) defined by Research Diagnostic Criteria. Two dimensions were identified among the four groups of 'affective' patients: the 'bipolar' and the 'schizophrenic' dimensions. They provided highly significant discrimination among the means of the four groups but were not very accurate in predicting group membership. The 'bipolar' dimension separates the UP from the BP and SBP, the SUP taking some intermediate value. The 'schizophrenic' dimension separates the BP and UP from the SUP, the SBP being intermediate. The two groups with the most similarities were the SBP and BP. The group with the most heterogeneity was the SUP, sharing similarities with the UP and SBP mostly. These conclusions are supported by results of familial aggregation on the same group of patients.