A 76-year-old man was admitted to our hospital in December of 1990, for investigation of progressive dyspnea on exertion over the previous 3 months. Physical examination revealed cyanosis, but no edema. Cardiomegaly was seen on chest X-ray, and findings compatible with right-sided congestive heart failure were revealed by ECG and echocardiography. Lung perfusion scintigrams showed multiple defects in both lungs, but no abnormal findings were detected on a ventilation study. Venograms of the lower extremities disclosed thrombosis of the right femoral vein. Therefore, a diagnosis of recurrent pulmonary thromboembolism was made. Furthermore, lupus anticoagulant and IgG-class anticardiolipin antibody were noted in the serum. The patient was treated with intravenous heparin, long-term warfarin, and transvenous placement of a Greenfield filter in the inferior vena cava. The clinical symptoms and signs improved, and no recurrence has been seen during the 13 months since the onset. On discharge, both antiphospholipid antibodies were negative. This case of primary antiphospholipid syndrome without autoimmune disease, with transiently positive antibodies only in the exacerbation phase, is discussed herein.