Nineteen children who presented with thromboses over a 7-year period were found to have a lupus anticoagulant (LA). The initial thrombosis was proximal deep vein thrombosis (DVT) in six children, central nervous system (CNS) in five, primary pulmonary in four, distal DVT in two, central venous in one, and proximal arterial in one. Five children were diagnosed with systemic lupus erythematosus (SLE), including two children for whom thrombosis was the presenting sign of SLE. The remaining 14 children were diagnosed with the antiphospholipid antibody (APA) syndrome. The APA syndrome was manifest by venous or arterial thrombosis in association with a positive LA; positive anticardiolipin antibodies and a fine, speckled antinuclear antibody (ANA) pattern were additionally found in the majority of children. Approximately one-half of the children with SLE or the APA syndrome had a pulmonary embolus, and one-half developed recurrent thrombosis. Oral anticoagulation with coumadin to achieve an INR of > 2.0 prevented thrombosis recurrence. The recognition of a LA in children with thrombosis necessitates evaluation for SLE, APA, and other autoantibodies.