We compared the concordance and predictive powers of activated partial thromboplastin time (APTT) and of IgG and IgM antibody to cardiolipin (aCL), for predicting fetal death in 50 pregnant women with systemic lupus erythematosus (SLE) and/or lupus anticoagulant. Overall concordance of any abnormal determination of aCL during pregnancy with any abnormal determination of APTT was 76% (0.05 less than p less than 0.10). Fetal death occurred in 6/12 (50%) of patients with high APTT compared to 5/20 (16%) of patients with low APTT; fetal death occurred in 10/13 (77%) of patients with abnormal aCL and in 2/37 (5%) of patients with normal aCL. Sensitivity for predicting fetal death was .55 for APTT and .85 for aCL; specificity was .81 for APTT and .92 for aCL. Abnormalities of APTT and aCL are sufficiently frequently discordant to prevent equation of the 2 assays. ACL is the better assay for predicting fetal death.