Radionuclide gated cardiac blood pool scan was used to quantify the left-sided valvular regurgitation by calculating the ratio of left ventricular to right ventricular stroke counts (the stroke count ratio) obtained directly from the "stroke volume image." The subjects consisted of 21 patients and 11 control subjects. In 21 patients, contrast angiography disclosed that 10 had isolated mitral regurgitation, nine had isolated aortic regurgitation and two had combined mitral and aortic regurgitation. The degree of aortic and/or mitral regurgitation was judged using the Sellers' criteria. The stroke count ratio was 1.34 +/- 0.21 (mean +/- SD) in a group without regurgitation, 1.73 +/- 0.13 in a group of patients with grade I, 2.14 +/- 0.19 in grade II, 2.76 +/- 0.58 in grade III and 5.04 +/- 0.27 in grade IV regurgitation. There was a good agreement between the stroke count ratios and quantitative angiographic estimates of regurgitation. Furthermore, the use of the "stroke volume image" (obtained from the subtraction of end-systolic from end-diastolic images) allowed a simplified drawing of the region of interest and avoided the problem in selecting the background. For its simplicity, we found little interobserver and intraobserver variations (r = 0.98 in the former and r = 0.99 in the latter). Therefore, this method permits a noninvasive and reliable assessment of the severity of left-sided valvular regurgitation, and has a great potential clinical utility in the long-term follow-up of asymptomatic patients with regurgitation, and in the evaluation of the results of medical and surgical interventions.