• A systematic interview regarding family history was administered to 48 men with bipolar affective illness who were attending a lithium clinic. Several families were found in which both the patient and his father had affective disorders, but the mother and maternal second-degree relatives were well. Of 30 men who had histories of hospitalization for mania, three had fathers with affective disorder (all bipolar). Of 18 men who had depression and hypomania, one father had unipolar depressive disorder. The hypothesis that bipolar manic-depressive illness may be transmitted by a single dominant genetic factor on the X chromosome is discussed in relation to these ill father—ill son pairs.