Pregnancy outcome was investigated: 1) in fifteen women suffering from idiopathic recurrent spontaneous abortion (RSA) and who had experienced pregnancies with two different partners, and 2) in the mothers, the sisters and wives of the brothers of 90 consecutively referred women with RSA. After adjusting for ascertainment bias, the patients with 2 partners had an abortion rate of 72% with the first spouse (significantly increased compared with the expected rate) and a nearly 100% abortion rate with the second spouse. This suggests that, in the main, RSA is not partner-specific. The sisters of the patients had suffered a significantly increased miscarriage rate (25.3%) compared with the observed rate in a Danish control group. The wives of the brothers had an abortion rate of 18.8%, which was not significantly increased vis-à-vis the controls. The sisters showed a lifetime incidence of RSA (10.6%) which was significantly greater than comparable estimates in the literature. These results suggest the existence of a familial predisposition to spontaneous abortion in families where RSA occurs.