The relation between lupus anticoagulant (LA) and repeated abortions was evaluated in a case-controlled study of 50 women with 3 or more unexplained spontaneous abortions compared with 50 control subjects who had 2 or more normal pregnancies and no previous spontaneous abortion or fetal deaths. LA was detected in 5 of 50 cases (10%, 95% confidence limits 1.69% to 18.31%) but in none of the 50 controls. There was an indication that missed abortions may be more frequent in LA positive women. The women who had recurrent abortions but were LA negative had prolonged prothrombin time values (though within normal range) compared to the control group. LA being a treatable cause of idiopathic recurrent abortion should be sought for in women with unexplained fetal losses.