The increase of coronary artery size in myocardial hypertrophy represents an adaptive mechanism to keep coronary blood flow normal. The relationship between coronary cross-sectional area and left ventricular muscle mass was determined angiographically in 10 patients with severe mitral regurgitation before and 28 +/- 15 months after successful mitral valve surgery. 10 subjects with atypical chest pain without coronary artery disease served as controls (C). Left ventricular muscle mass was increased preoperatively in mitral regurgitation (257 g vs C = 129 g; p < 0.001) and decreased postoperatively (205 g; p < 0.01 vs preop. and vs C). The cross-sectional area of the left coronary (= left anterior descending+left circumflex) artery was augmented preoperatively (26.5 vs C = 14.0 mm2; p < 0.001) and decreased postoperatively (22.9 mm2; p < 0.05 vs preop. and vs C). The cross-sectional area of the left coronary artery per 100 g LV muscle mass was not different in the three groups. The cross-sectional area of the right coronary artery was also increased before surgery (12.7 vs C = 8.8 mm2; p < 0.05) and decreased postoperatively (11.3 mm2; p < 0.05 vs preop. ns vs C). Our data show that in mitral regurgitation the size of the left coronary artery increases proportionally to the increase in left ventricular muscle mass. Also, the right coronary artery shows slight enlargement which is probably due to the pressure overload of the right ventricle. After surgery there is regression but not normalization of the size of the coronary arteries.