OBJECTIVE To determine the prevalence of cardiac valvular involvement in patients with the primary antiphospholipid syndrome. DESIGN Cross-sectional study with evaluation of case patients and control patients by Doppler echocardiography. The mean follow-up for case patients was 22 months. SETTING University-based tertiary medical center. PATIENTS Twenty-eight consecutive patients who were diagnosed with the primary antiphospholipid syndrome during a 10-year period; 28 age- and sex-matched healthy controls. MEASUREMENTS AND MAIN RESULTS Ten patients (36%; 95% Cl, 19% to 56%) with the primary antiphospholipid syndrome had cardiac valvular involvement: Four patients had mitral valve involvement; four patients, aortic valve involvement; and two patients, both mitral and aortic valvular involvement; no patients had tricuspid or pulmonary valve disease. Eight of 10 patients had a regurgitant murmur. None of the control patients had valvular disease. The mean mitral valve thickness in patients with mitral valve involvement was 7.0 +/- 1.6 mm, compared with 2.7 +/- 0.8 mm in patients with normal valves and 3.2 +/- 0.9 mm in the control group. The mean aortic valve thickness in patients with aortic valve involvement was 3.8 +/- 0.5 mm compared with 1.4 +/- 0.3 mm in patients with normal valves and 1.4 +/- 0.5 mm in the control group. Stenotic lesions were not found. Regurgitation was severe in two patients (one required surgery), moderate in three patients, and mild in three patients. CONCLUSIONS Valvular involvement is frequently found in patients with the primary antiphospholipid syndrome. The lesions are left-sided, causing regurgitation that may be clinically important.