Over a 3-year period, we studied 43 women who presented with severe preeclampsia prior to 34 weeks' gestation. Seven (16%) had significant levels of antiphospholipid antibodies, whereas none of the normotensive controls of similar gestational age had antiphospholipid antibodies (P less than .001). Three of the seven women with antiphospholipid antibodies suffered the following complications during the peripartum period: 1) cerebral infarction and episodes of transient monocular blindness; 2) pulmonary embolism, deep venous thrombosis, and an autoimmune flare in the postpartum period; and 3) transient monocular blindness and amnesia after delivery. Our experience suggests that antiphospholipid antibodies are found in a substantial proportion of cases of early-onset severe preeclampsia and have important clinical implications. We suggest that patients with early-onset severe preeclampsia be screened for antiphospholipid antibodies; if antibodies are detected, these women should be considered for prophylactic anticoagulation therapy.