Age-of-onset data were gathered on first-degree relatives of 252 probands with bipolar and unipolar affective disorders. Early onset probands (younger than 40 at onset) had more early onset relatives and a greater risk for affective disorder among their relatives than late onset probands (40 or older). This indicates that age-of-onset is a familial factor correlated with the liability to affective illness. Multiple threshold models of inheritance were applied to the data using age-of-onset as a liability-threshold determinant. The hypothesis of autosomal single-major locus was ruled out. Multifactorial-polygenic inheritance provided a better fit to the data. The data suggest that early and late onset affective disorders can be placed at different thresholds on a genetic environmental continuum and that the early onset form is more deviant genetically than the late onset type. The implications for genetic research in affective disorder are discussed.