To investigate the contribution of genetic and environmental factors in the etiology of mood disorders, a study was initiated to examine the frequency of psychiatric disorders in the biological and adoptive relatives of adult adoptees with mood disorders and in matched normal adoptees. Psychiatric evaluations of the relatives were made on the basis of independent blind diagnoses based on mental hospital and other official records. Analysis of the data showed an eightfold increase in unipolar depression among the biological relatives of the index cases and a 15-fold increase in suicide among the biological relatives of the index cases. These data demonstrate a significant genetic contribution to unipolar depression and suicide. They fail to disclose a significant contribution of family-associated transmission in the genesis of the mood disorders.