The levels of anticardiolipin antibodies (ACA) and lupus anticoagulant (LAC) were measured in 266 consecutive unselected patients with a history of venous thrombosis. 19 (7.1%) had elevated levels of IgG- or IgM-ACA. The prevalence of LAC was 1 of 266 (0.4%) in the whole patient group and 1 of 19 (5.3%) in the ACA-positive group. Patients with elevated ACA levels did not differ from those with normal ACA with regard to age at the first thrombosis, risk of recurrence, presence of arterial thrombosis, and other clinical features. 8 out of 19 (42.1%) patients with elevated ACA levels also had elevated levels of antinuclear antibodies, but only 1 fulfilled the criteria of systemic lupus erythematosus. These data indicate that in some patients with elevated ACA, autoimmune processes may be present. The clinical significance of elevated ACA levels is uncertain.