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Results of the present study provide evidence of: 1) a positive association between bipolar affective disorder and blood type O and a corresponding negative association between the former and blood type A, 2) a positive association between unipolar affective disorder and blood type O, and 3) a positive association between involutional depression and blood type A and a corresponding negative association between the former and blood types B and O. Sex does not appear to modify the ABO blood types' distribution in patients with bipolar, unipolar affective disorder, or involutional depression, and the same holds for early- or late-onset of the illness in patients with bipolar or unipolar affective disorders. Findings in the present study do not support the validity of the bipolar-unipolar distinction of affective disorders, and provide evidence in favour of the view that involutional depression is a genetically distinct nosological entity.