The antiphospholipid syndrome--the association of venous and/or arterial thromboses, often accompanied by thrombocytopenia in the presence of the antiphospholipid antibodies ("lupus anticoagulant" antibodies to cardiolipin)--is seen mainly in patients with systemic lupus erythematosus (SLE) and the closely related "lupus-like" disease, i.e., lupus patients not conforming to the 1982 revised American Rheumatism Association classification for SLE. It is also seen in a group of patients who do not manifest any of the major clinical or serologic features of SLE, the majority of whom do not appear to progress to classical lupus. A multicenter study of 70 of these patients is documented and their major clinical and serologic characteristics examined: They have been characterized as suffering from a "primary" antiphospholipid syndrome and present typically with a history of deep vein thromboses, often accompanied by pulmonary thromboembolism, which in a few is complicated by thromboembolic pulmonary hypertension, arterial occlusions (most commonly strokes), or fetal loss. The events are often recurrent and may be accompanied by hemocytopenias (thrombocytopenia and less frequently Coombs positivity and/or hemolytic anemia). They are often antinuclear antibody-negative and are always negative for antibodies to dsDNA and to ENA, typical serologic features of SLE. There may be a family history of SLE or a familial clotting tendency in a minority. The group of patients presented appears to be closely related, but distinctly separate from SLE.