To define the range of cardiac involvement in the Churg-Strauss syndrome, M mode, continuous wave Doppler, and cross sectional echocardiograms were recorded in twelve patients with the disorder. The M mode recordings were digitised and the cross sectional images were recorded with standardised gain settings to determine regional myocardial echo amplitude. Left ventricular end diastolic and end systolic dimensions were increased above the normal 95% confidence interval in four patients, three of whom showed a depressed shortening fraction. Mitral regurgitation was present in six patients; this was severe enough to need valve replacement in two. Mean echo amplitude in both the septum and the posterior wall was significantly increased above normal by a mean (SD) of 4.87 (2.57) dB, suggesting the presence of myocardial fibrosis. There was no evidence of subendocardial involvement as there is in other hypereosinophilic syndromes. Mitral regurgitation is common in the Churg-Strauss syndrome. This cannot be ascribed to involvement of the cusps or chordae and it occurs even when ventricular function is well preserved. It is suggested that mitral regurgitation is caused by diffuse myocardial fibrosis.