Sydenham's chorea is often encountered in pure form, but it may also occur in association with carditis. Recently, it has been reported that silent, mild valvular regurgitation has been encountered in patients with rheumatic fever manifested by isolated polyarthritis and pure chorea. We studied 22 children with Sydenham's chorea to determine clinical and subclinical heart involvement at the time of the initial diagnosis. Ages of the patients ranged from 6-19 y (mean 13.14 +/- 3.0), and the female/male ratio was 3.4:1. All patients were evaluated by clinical and echocardiographic examination with respect to heart involvement. Carditis was detected in only 5 of 22 patients (23%) by auscultation. Clinical valvular involvement was determined. mitral regurgitation in three and combined mitral and aortic regurgitation in two. The frequency of valvular involvement increased significantly to 63% (14/22) after adding silent mitral regurgitation detected by colour Doppler echocardiography (chi(2) = 7.50, p = 0.0062). We found that clinically inaudible, but echocardiographically significant, mild valvular regurgitation was frequent in patients with pure chorea. It is concluded that colour Doppler echocardiography may be useful in detecting silent valvular regurgitation and in deciding the duration of prophylaxis.