• In a family study of 1,254 adult relatives of patients and controls, lifetime prevalences of major affective disorder (including schizoaffective) were 37%, 24%, 25%, 20% and 7% in relatives of probands with schizoaffective, bipolar I, bipolar II, and unipolar disease, and normal controls. These data were compatible with the different affective disorders representing thresholds on a continuum of underlying multifactorial vulnerability. In this model, schizoaffective illness represents greatest vulnerability, followed by bipolar I and bipolar II, then unipolar illnesses. Alcoholism, drug abuse, and sociopathy were not more frequent in relatives of patients v relatives of controls. Sex-related transmission of morbid risk was not present. Morbid risk was 74% to offspring of two ill parents, and 27% to offspring of one ill parent. Nationality and age at time of interview seem to be nongenetic factors that affect frequency of diagnosis.