Abstract: Anti-cardiolipin antibodies (ACA) occur in patients with SLE, lupus-like illness and with some other illnesses, such as malignancy, syphilis and among healthy elderly. We found that the prevalence of IgG and IgM ACA isotopes in 147 Iranian SLE patients was 49.7% for IgG and 5.6% for IgM. In addition, some SLE and non-SLE patients from other centers were studied: the prevalence of ACA-IgG in SLE patients from Mashad was 54%; among patients of another center in Tehran, it was 26% and 22%, in MCTDs, it was 16%, in preeclampsia, 54%, in neurologic patients with positive-FTA, 60%, in post-M1 patients, it was only 5%. In 147 SLE patients followed at Tehran University of Med. Sci., a statistically significant association exists between ACA and history of thrombotic event (P=0.0002), cardiac disorder (P=0.0021), recurrent abortion (P=0.0045), thrombocytopenia (P=0.0001), VDRL (P=0.0001), CRP (P=0.001), abnormal PTT (P=0.01), neuropsychiatric disorders (P=0.25) and very high ESR. According to results of this study, it is concluded that lupus patients with increased ACA are at increased risk of recurrent abortion, thrombocytopenia, cardiac disease, and other complications.