A group of 46 bipolar probands and their first degree relatives were studied. A high rate of affective disorder (19.6 per cent) was found, including both unipolar (13.2 per cent) and bipolar (6.4 per cent) types, with females predominating (3:1). The presence of four father-son pairs suffering from affective disorder made the hypothesis of X-linked dominance untenable. Results compatible with polygenic inheritance were found, using both Slater's and Falconer's methods. There was no evidence for assortative mating or for increased total number of females (both well and ill) among first degree relatives. The probands and affectively ill first degree relatives who have died show an alarmingly high rate of suicide (46 per cent). Other forms of mental disorder, including alcoholism, were no more common than in the rest of the community.