A prospective echocardiographic study was carried out on 55 patients with the recently described 'primary' antiphospholipid syndrome derived from three university medical centres. The prevalence of valvular lesions in patients with this syndrome was 38% compared with 4% in a control group of 55 healthy volunteers (P < 0.001). Mean age of patients with valve abnormalities was 42 +/- 12 years and of those without, 30 +/- 10 years (P < 0.05). One patient had a morphologic echocardiographic pattern suggestive of non-infective verrucous mitral endocarditis. Twenty patients had a two-dimensional or Doppler echocardiographic pattern of significant valvular dysfunction--either regurgitation or stenosis--without evidence of vegetations. Mitral and aortic regurgitation were the most common lesions in these patients. During follow-up of patients with valvular disease, haemodynamically significant clinical valve disease developed in four and surgery was required in one. Eleven patients had cerebrovascular occlusions. Thus, valvular heart disease, particularly affecting the mitral and aortic valves, is common in patients with the 'primary' antiphospholipid syndrome, especially in those over 40 years old.