The interest in autoimmune pregnancy loss has increased recently with the discovery of the association between the presence of antibodies to phospholipids and cardiolipin and habitual abortion. To evaluate the prevalence of anticardiolipin antibodies in patients with recurrent fetal loss and the effect of low-dose aspirin and corticosteroid treatment, we examined 67 women for anticardiolipin antibodies with an enzyme-linked immunosorbent assay. Increased levels of anticardiolipin antibodies were found in 34 of these women (50.7%) and in none of 12 normal pregnant controls. Seventeen of these 34 patients conceived and were treated with aspirin, 100 mg/day, and fluocortolone, 10 mg/day. Twelve gravidas reached term, all delivering live infants with good Apgar scores and normal birth weights. Three others delivered prematurely, two of them because of premature rupture of membranes and the other because of severe fetal growth retardation. This latter woman delivered at 31 weeks, and the 1000-g infant died neonatally. The neonatal survival rate was 82% in the treated patients, versus 18% in the same group of subjects in their previous, untreated gestations. The umbilical artery resistance index of the treated subjects was not different from that of anticardiolipin-negative hypertensive women who did not receive aspirin and corticosteroid therapy. However, the uterine artery resistance index was significantly lower in the hypertensive treated group than in the untreated group. Whether this difference reflected a beneficial effect of low-dose aspirin and corticosteroid therapy on pregnancy outcome and survival rate remains to be evaluated.