Bunazosin hydrochloride (Detantol) was administrated in 3-mg oral doses to 10 patients (6 males and 4 females, average 49.9 years) with dilated cardiomyopathy (DCM) of class II in the NYHA functional classification. The patients were observed for changes in their hemodynamics at rest, chest X-ray, echocardiography, humoral factors and symptom-limited exercise tolerance with treadmill testing before treatment and 4 and 8 weeks after administration. All but the left ventricular end-diastolic diameter after 8 weeks were unchanged. Of the humoral factors, only aldosterone decreased significantly during the treatment. On the other hand, the duration of symptom-limited exercise was prolonged significantly, and for a given exercise load, a reduction of heart rate and systolic blood pressure as well as a drop in the pressure-rate product occurred at 8 weeks after administration. Therefore, bunazosin appears to improve cardiac function and exercise tolerance in the patients with DCM, at dosages which do not affect hemodynamics at rest.