To evaluate cardiac involvement in primary antiphospholipid syndrome, two-dimensional and Doppler echocardiographic studies were performed in 34 consecutive patients with this syndrome. All patients had an increased level of serum anticardiolipin antibodies with no evidence of malignancy or systemic lupus erythematosus. The clinical manifestations of primary antiphospholipid syndrome were arterial thrombosis in 14 patients, venous thrombosis in 6 and recurrent fetal loss in 14. Valvular lesions were observed on two-dimensional echocardiography in 11 patients (32%) (9 women and 2 men), aged 24 to 57 years (mean +/- 1 SD 36 +/- 10). Abnormal echocardiographic findings were observed in 9 (64%) of 14 patients with arterial thrombosis versus 1 (17%) of 6 patients with venous thrombosis and 1 (7%) of 14 patients with recurrent fetal loss. The most common echocardiographic abnormality was mitral leaflet thickening, found in five patients; this was associated with mitral regurgitation in three and with combined mild mitral stenosis and regurgitation in one patient. Localized subvalvular mitral thickening was observed in one patient and calcification of the anulus in another. Aortic valve thickening was observed in two patients, one of whom also had a moderate degree of aortic regurgitation. Vegetation-like lesions on the mitral or aortic valve were found in two patients. It is concluded that valvular lesions are commonly found in primary antiphospholipid syndrome, particularly when the syndrome is manifested by peripheral arterial thrombosis. The location and appearance of valvular lesions in this syndrome are heterogeneous. Most patients have no clinically significant valvular disease. Two-dimensional and Doppler echocardiographic studies are often informative in these patients.