Assortative mating among 56 married in-patients with primary affective disorders and their spouses was studied by determining the prevalence of psychiatric illness among the spouses by means of direct interviews and standardized diagnostic criteria. A high degree of assortative mating among both male and female patients was observed for total psychiatric illness, broad spectrum affective illness and major depression. A significantly higher prevalence of psychiatric and affective illness was found among the first-degree relatives of the ill spouses when compared with the first-degree relatives of the well spouses. There was a high degree of diagnostic concordance between the patients and spouses for both affective illness and alcoholism, with a higher degree of assortative mating among bipolar patients than among unipolar patients. The finding in this study of an increased prevalence of psychiatric disorder in the first-degree relatives of the ill spouses would support the hypothesis that there is a tendency for individuals with a predisposition to psychiatric illness to marry, rather than the existence of a marital interaction which causes an increased concordance for psychiatric illness.