We studied a group of 59 unselected patients with systemic lupus erythematosus (SLE); these patients were from a defined population who lived in southern Sweden. We found that serum concentrations of anticardiolipin antibodies were increased in 32 SLE patients (54.2%). No significant correlation between increased amounts of anticardiolipin antibodies and clinical symptoms, such as thrombocytopenia or thrombosis, was found. Serial serum samples from 28 patients (12 patients were from the epidemiologic cohort) were analyzed. Sixteen of these 28 patients (57.1%) had increased levels of anticardiolipin antibodies; in most cases, there was no variation in these values with regard to clinical disease flares or treatment. Increased concentrations of anticardiolipin were observed in 4 patients with cerebral infarction. However, very high concentrations of anticardiolipin antibodies were observed in several patients with inactive SLE who had no history of thrombosis or thrombocytopenia. Our results underscore the importance of studying unselected patient groups when correlating laboratory data with clinical manifestations of disease.