Abstract: The determination of serum anti-cardiolipin antibodies （ACA） by ELISA among 272 patients with various diseases and 81 normal controls was reported and its clinical significance was evaluated. The results showed that the positive rates of ACA in active SLE, RF positive RA, SS, PSS and infectious diseases groups were 66%, 38%, 100%, 50% and 44%for IgG; and 53%, 46%, 20%, 50% and 42% for IgG respectively. In contrast, the positive rates of both IgG and IgM-ACA in inactive SLE, RF negative arthritis and othor disease control groups were much lower, without significant differences from normal controls. ACAS positivity were related with the severity and activity of the disease in patients with SLE, RA, SS, and PSS. The thrombocytopenia in SLE, the extraarticular manifestions in RA, the articular and cutaneous lesions of Lyme disease were showed to be closely related with the high levels of ACA. The results also suggested that ACAs couid be used as a diagnostic tool for some infectious diseases and as a guide in therapy.