Bipolar disorder and alcoholism are familial disorders. The familial–genetic relationship between both is controversial and has received insufficient study. This study explores whether bipolar disorder and alcoholism share familial risk factors, and whether the co-occurrence of lifetime diagnosis of bipolar disorder and alcoholism is familial. We report on first-degree relatives of 146 consecutively admitted patients with either bipolar disorder or/and alcoholism; relatives of the patients (in total 728 relatives directly interviewed) were compared with first-degree relatives of 109 general population probands (320 relatives directly interviewed). Overlap between the familial components underlying bipolar disorder and alcoholism was not observed if the analysis was restricted to ‘pure’ diagnostic groups. Excess comorbidity between bipolar disorder and alcoholism was observed in relatives. Multiple sources for this excess of comorbidity between major affective disorders and alcoholism in families of probands with bipolar disorder are likely; in particular, we found evidence for a distinct subgroup of comorbid cases with familial comorbidity; however, excess comorbidity was also found in absence of familial loading with alcoholism.