In this study, we clarified the clinical significance of IgG anti-phosphatidylserine-prothrombin complex (PS-PT) antibodies in the diagnosis of antiphospholipid syndrome (APS). The study population consisted of 113 patients with SLE and lupus-like disease. IgG anti-PS-PT antibodies were examined by ELISA. These antibodies were detected in 31% of the population, and were significantly associated with thrombosis and fetal loss. The frequency was significantly higher in patients who did not satisfy the Sapporo criteria, but met the Harris and Hughes diagnostic criteria (36%) than in non-APS patients (14%). The above findings suggest that IgG anti-PS-PT antibodies should be listed in the diagnostic criteria for APS.