Antiphospholipid syndrome, a condition characterized by one or more thrombotic or pregnancy-related clinical features in association with medium to high levels of antiphospholipid antibodies, has emerged as an important diagnostic consideration in several medical fields. Antiphospholipid syndrome is one of the few treatable causes of pregnancy loss, and successful pregnancy rates of 70% or more can be achieved with appropriate treatment. Heparin, usually combined with low-dose aspirin, is used in patients at risk for thrombosis. Pregnancy in these women is associated with increased rates of preeclampsia, placental insufficiency, and preterm delivery, so that attentive clinical care is required for best outcomes. Recent studies indicate that women at low risk for thrombosis may be treated with low-dose aspirin. However, remaining controversies and unanswered questions in the field of antiphospholipid syndrome are a source of clinical confusion. This review highlights the most important controversies, taking into account the results of recent obstetric treatment trials and our own clinical experience. (C) 2003 by The American College of Obstetricians and Gynecologists.