The relationship between left ventricular (LV) shape and functional mitral regurgitation (MR) was examined in 18 dogs with long-standing heart failure produced by multiple sequential intracoronary microembolizations. Global LV shape changes were evaluated from angiographic silhouettes obtained at baseline (before embolization) and during heart failure. LV shape changes at end systole and end diastole were quantitated based upon the ratio of the major-to-minor axis and the sphericity index. Among the 18 dogs studied, 11 developed 1+ to 3+ MR during heart failure and seven did not. There was no difference among the two groups with respect to hemodynamics, LV ejection fraction, chamber volume, and regional wall motion. A significant difference, however, was observed between the two groups with respect to the global indexes of LV shape. In dogs with MR, the end-systolic major-to-minor axis ratio decreased 29 +/- 3% between baseline and heart failure compared with only a 16 +/- 3% decrease in dogs without MR (p less than 0.01). In dogs with MR, the end-systolic sphericity index increased 72 +/- 11% between baseline and heart failure compared with an increase of only 30 +/- 5% in dogs without MR (p less than 0.02). Significant and directionally similar differences were observed during end diastole. These data indicate that in heart failure functional MR is associated with a more spherical LV and is not the result of simple LV chamber enlargement. Shape changes of the LV that occur in heart failure may be an important determinant of functional MR.