Although mechanisms that prevent rejection of the conceptus are incompletely understood, recent evidence suggests that maternal immunologic aberrations may cause repeated abortions. Autoimmune conditions associated with antiphospholipid antibodies sometimes produce vascular abnormalities in the decidua and placenta; successful pregnancies can be achieved in most of these women by treatment with corticosteroids and low-dose aspirin. Abnormal maternal immune responses to paternal or trophoblast alloantigens, with insufficient production of blocking antibodies or suppressor cells, have also been implicated. Immunization of these patients with paternal or third-party leukocytes has resulted in a number of live births. However, the mechanism of action, effectiveness, and safety of all treatment regimens remain controversial. Present recommendations for the evaluation of recurrent-abortion patients include diagnosis and appropriate treatment of traditional nonimmunologic and more recently recognized autoimmune factors. The remaining patients with no detectable cause for repetitive pregnancy loss are candidates for referral to research centers for further immunologic evaluation and experimental immunotherapy.