In a controlled, prospective study of 44 consecutive women with idiopathic habitual abortion, only 5% had symptoms of rheumatic disease. Patients did not differ from control subjects in the frequency of positive results on tests for antinuclear antibody or anti-double-stranded DNA. Levels of C3 and C4 were higher in the habitual aborters. No patients had anti-Ro. The antiphospholipid antibody results were analyzed using 2 methods: the frequency of antiphospholipid antibodies was 9% by lupus anticoagulant using the Russell viper venom time (95% confidence interval 22-2.5) and 11% by anticardiolipin antibody assay (95% confidence interval 25-3.7), which was not significantly different from that in control subjects. However, the mean levels in the aborters (although within the normal range) were significantly higher than those in control subjects for anti-double-stranded DNA (P = 0.004), lupus anticoagulant (by Russell viper venom time; P = 0.05), and anticardiolipin antibody (P = 0.0007), when examined by multiple linear regression analysis corrected for age and concurrent pregnancy. Of the 3 patients with antiphospholipid antibodies and subsequent successful pregnancies, only 1 was treated with prednisone and aspirin. We conclude that, in the majority of women, subclinical lupus, anti-Ro, the lupus anticoagulant, and anticardiolipin antibodies are not associated with idiopathic habitual abortion.