SummaryThirty years ago it was suggested that the apparently higher incidence of manic-depressive illness in women might be due to X-linked heredity. The hypothesis was undermined by subsequent reports of the frequent occurrence of father to son transmission. Winokur and his associates recently revived it, providing data which indicated that such transmission is absent or rare in the bipolar form of the illness. Additional support has come from linkage studies with known genetic markers located on the X chromosome. The present study, based on the 400 parents of 100 male and 100 female bipolar manic-depressive probands, failed to discover a lack of father-son compared to other affected parent-child pairs. This finding, together with a review of the literature, would indicate that it is premature to invoke X-linked heredity as a general explanation for bipolar manic-depressive illness, though there is mounting evidence that it may account for the illness in some family pedigrees.