Thirty male manic probands and their first-degree relatives were investigated by means of a systematic psychiatric interview in order to determine the frequency of familial illness and possible mode of transmission of bipolar manic-depressive disorder. The infrequency of male-to-male transmission of affective disorder and the relative preponderance of the illness on the maternal side of the family both support similar previous findings that are suggestive of sex linkage in this illness. The incidence of alcoholism was found to be high and more prevalent on the paternal side of the families.