Patients with systemic lupus erythematosus (SLE) are at risk of developing deep venous thrombosis (DVT). Should anticardiolipin antibodies (aCL) be detectable, this risk is significantly raised, particularly when these autoanti-bodies are cofactor-dependent. We conducted a cross-sectional study of consecutive unselected outpatients referred for clinical suspicion of DVT, as an attempt to address the following questions: firstly, were aCL antibodies associated with DVT in non-SLE patients? Secondly, was this association related to the cofactor dependence? From March 1992 to February 1994, 208 patients were enrolled in the study. Venography was positive in 110 patients (DVT patients), while the diagnosis of DVT could not be confirmed in the remaining 98 (referred to as disease controls). ACL was measured by ELISA, for IgG and IgM isotypes in two ways: fetal calf serum or bovine serum albumin were used as blocking agents to distinguish between cofactor-dependent and cofactor-independent antibodies. Positive aCL was defined as optical density (OD) values greater than the 95th percentile of OD distribution of 60 healthy controls. We found a high frequency of positive IgG aCL antibodies in both DVT patients and in disease controls (25.5 vs 23.5%). We suggest an association between IgM aCL and DVT. This association was, however, not dependent on the cofactor requirement.