In patients with systemic lupus erythematosus (SLE), the synthesis of antibodies to cardiolipin (A-CL) is associated with the development of venous and arterial thromboses localized in minor and middle-sized vessels, in the venous system and capillaries. The determination of various A-CL isotypes may be used to predict thromboses in SLE patients. Overall 210 patients (185 women and 25 men) with a verified diagnosis of SLE were examined. The patients were not screened in accordance with some or other clinical signs of the antiphospholipid syndrome. The control group comprised 100 healthy subjects (donors). The IgG, IgA and IgM isotypes of A-CL were determined by ELISA. Sera of SLE patients showed an increase of the concentration of A-CL of both certain isotypes and their potential combinations. Among A-CL-positive patients, the IgG isotype of A-CL was detected in 75% of cases, the IgM isotype of A-CL in 61%, and the IgA isotype of A-CL in 36% of cases. Thrombotic complications were recorded in 19% of patients. They were induced by hyperproduction of the three combinations of the A-CL isotypes: A-CL IgM, IgM+IgA, and IgG+IgA+IgM. Patients whose sera contained A-CL of all three types at a time were most prone to thrombotic complications. It has turned out that the percentage of SLE patients with thromboses was higher than that of SLE patients without thromboses, starting from the definite A-CL concentration (21 GPL).(ABSTRACT TRUNCATED AT 250 WORDS)