Lupus anticoagulant, anticardiolipin, antinuclear, anti-deoxyribonucleic acid, antithyroglobulin, and antithyroid microsomal antibodies were assayed during third-trimester pregnancy (100 normal, 100 with complications). In spite of a normal activated partial thromboplastin time in all instances, lupus anticoagulant was further investigated by three additional procedures: tissue thromboplastin inhibition time, platelet neutralization procedure, and cephalin neutralization test. The prevalence of autoantibodies in pregnancies with hypertension reaches 16% (four with lupus anticoagulant, two with anticardiolipin, and two with antithyroid microsomal antibodies), which is significantly greater than that for idiopathic fetal growth retardation (2%) (one with lupus anticoagulant antibodies) and normal pregnancies (3%) (two with antithyroglobulin and one with autithyroid microsomal antibodies) (p less than 0.01). Autoantibodies were equally distributed between patients with gestational hypertension and those with preeclampsia. When compared with the 42 patients with hypertension and no autoantibodies, the eight patients with autoantibody had a more frequent history of fetal growth retardation (p less than 0.05), but there was no difference in the severity of hypertension, the frequency of obstetric complications, or the outcome of pregnancy. They did not require any specific treatment.