Eight hundred twenty-three relatives of bipolar and schizoaffective patients were divided into groups of birth cohorts. The rates of bipolar, schizoaffective, and unipolar disorders were higher in the cohorts born after 1940 than in the cohorts born earlier. Life-table analysis of the cumulative hazard of illness in the different cohorts revealed significant differences among them, implying that the cumulative hazard of developing any affective disorder by a given age was greater in the post-1940 cohorts. When these data are combined with other reports, an ominous trend may be present, leading to an increase in prevalence of a broad spectrum of familial affective disorders in the coming decades.