Twenty-one patients with anti-phospholipid antibodies (APLA), who were selected from 104 patients with antinuclear antibody or anti-DNA antibody, were studied to define clinical characteristics of APLA. Of the 21 patients, the incidences of IgG anti-cardiolipin antibody (ACA), IgM ACA, lupus anticoagulant (LAC) and BFP-STS were 20, 7, 11 and 12, respectively, and they were highly related with each other. The number of items of the 1982 ARA revised criteria for the classification of SLE was significantly low (mean value = 3.3), and also the level of serum C 4 was significantly high (mean value = 88% of normal) compared with those in patients without APLA. The incidence of thrombocytopenia and hemolysis was significantly high when compared with those in patients without APLA, and they were closely related to the presence of LAC. The incidence of thrombosis was markedly high (48%), 10 of 21 patients, especially those of cerebral infarction and deep vein thrombosis. Cerebral infarction was significantly associated with LAC. The obstetric complication was 4 of 13 patients (31%) in the incidence, which was significantly high compared with that in patients without APLA. Of them, natural abortion was closely associated with LAC. Based on these observations, it seems that autoimmunological disease activity in patients with APLA may be slight to mild, and strongly suggested that APLA may play an important role particularly in the pathogenesis of acquired thrombotic tendency.