Development of mitral regurgitation (MR) during acute myocardial ischemia is a well known occurrence. To assess the feasibility and clinical utility of detecting MR during exercise testing, color Doppler examinations were performed in 22 patients with angiographically proved coronary artery disease (CAD) and in 17 normal subjects before, during and after graded supine bicycle exercise. Not only was MR visualized using color Doppler during dynamic testing, but it was also slightly more sensitive (59% vs 54%) and specific (100% vs 88%) than the electrocardiographic response in identifying patients with CAD. When the appearance of MR or diagnostic electrocardiographic changes or both were used to identify patients with CAD, the sensitivity of exercise tolerance testing increased to 82%, although the specificity was 88%. In addition, exercise-induced MR was observed to be as sensitive and specific as exercise-induced wall motion abnormalities. Combining exercise-induced MR with wall motion abnormalities also increased the sensitivity to 82%, with the specificity remaining at 100%. With use of exercise-induced MR, wall motion abnormalities or electrocardiographic changes, the sensitivity and specificity of the exercise test in diagnosing CAD was 91% and 88%, respectively. The degree of MR as estimated by maximal area of regurgitation signals, as well as by its ratio to left atrial area, did not correlate with extent of CAD. However, the presence of exercise-induced MR suggested an increased likelihood of 3-vessel CAD because it was found in 9 of 11 patients with 3-vessel CAD, compared with 2 of 5 patients with 2-vessel and 2 of 6 patients with 1-vessel CAD. Thus, assessment of MR by color Doppler during exercise testing may provide an additional diagnostic criterion for detecting patients with CAD. © 1987.