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During a prospective study of pregnancies in women with systemic lupus erythematosus, we examined the relation between antibody to cardiolipin, measured by the enzyme-linked immunosorbent assay, and midpregnancy fetal distress, identified by abnormal results of antepartum fetal heart-rate testing or by fetal death. All of nine patients with lupus and this complication had abnormally high antibody levels (mean, 212.3 +/- 55.3 units), as compared with values in normal nonpregnant women (28.2 +/- 10.1 units). None of 12 pregnant patients with lupus but without this complication had antibody levels above 50 units (mean, 27.5 +/- 3.4 units; P less than 0.005 vs. women with lupus and fetal distress); 4 of 12 pregnant subjects without lupus had antibody levels above 50 units (mean, 42.5 +/- 11.0), and fetal death occurred in the subject with the highest level. The mean antibody level in 12 nonpregnant patients with lupus was 117.4 +/- 35.0 units. Two patients who had lupus anticoagulant but not clinical lupus, both with histories of prior fetal death, also had high antibody levels; fetal death occurred in one, and spontaneous fetal bradycardia in the other. Antibody to cardiolipin was loosely linked to a history, but not the simultaneous presence, of demonstrable lupus anticoagulant or thrombocytopenia, and could be detected as early in pregnancy as either anticoagulant or thrombocytopenia. We conclude that measurement of antibody to cardiolipin is the most sensitive assay to predict fetal distress or death in patients with systemic lupus erythematosus and may be of pathogenetic importance in this syndrome.