A previous analysis found a relatively high rate of alcoholism in a cohort of bipolar I subjects, and a trend for increased rates of alcoholism in relatives of subjects with both bipolar I disorder and alcoholism, compared to relatives of subjects with bipolar I disorder and no alcoholism. The sample of subjects with bipolar I disorder has been enlarged through continued follow-up, permitting new analyses to address the association and heritability of bipolar I disorder with alcoholism. Probands with bipolar I disorder were followed for 10 years as part of the NIMH Collaborative Depression Study. The rate of alcoholism in relatives of probands with both bipolar I disorder and alcoholism was compared to the rate of alcoholism in relatives of probands with bipolar disorder and no alcoholism. The prevalence of alcoholism in relatives of subjects with bipolar I disorder was compared to the rate of alcoholism in relatives of control subjects. Relatives of probands with bipolar I disorder showed a higher rate of alcoholism than relatives of controls. Relatives of probands with bipolar I disorder and alcoholism showed a higher rate of alcoholism than relatives of probands with bipolar I disorder without alcoholism. These data suggest that familial alcoholism may contribute to a vulnerability to bipolar I disorder, and that there is a shared heritability for the two disorders.