Coronary sinus blood flow (CSBF) was measured by the dye dilution method in 18 patients with idiopathic cardiomyopathy including 13 with hypertrophic type and 5 with congestive type, and the capacity for increasing CSBF in response to handgrip exercise was studied in 14 patients. CSBF at rest ranged from 41 to 236ml/min/M². Average CSBF of 114±12ml/min/M² (112±14 in hypertrophic type and 119±25 in congestive type) was significantly larger than control value of 77±6 presented previously (p<0.01). Handgrip exercise at 30% maximal effort for 3min resulted in the increase in CSBF, averaging 23±8 ml/min/M² (22±8%). However, in about one-half of cases, the percent increase in CSBF was much smaller than the percent increase in effort index which represents myocardial oxygen demand. The change of CSBF was not correlated with the changes of left ventricular systolic pressure, cardiac output, and stroke work. The diminished response of CSBF to isometric handgrip exercise was ascribed to the decreased coronary vascular reserve in this disorder.