Factors affecting the exercise capacity of patients with mitral stenosis (MS) and aortic regurgitation (AR) are incompletely understood. Accordingly, exercise capacity was assessed in 13 patients with MS and in 13 with AR by means of cardiopulmonary exercise testing before as well as 3, 6 and 12 months after valve replacement. Left- and right-ventricular function were evaluated echocardiographically. Both in MS and in AR exercise capacity expressed by maximal oxygen consumption (VO<sub>2max</sub>) increased significantly after valve replacement and was directly related to right ventricular (RV) function assessed by the ratio of RV acceleration time to RV ejection time (r = 0.87, p < 0.001 and r = 0.74, p < 0.001, respectively) and inversely related to left atrial diameter (r = -0.72, p < 0.001 and r = -0.76, p < 0.001, respectively). No relation between VO<sub>2max</sub> and resting left-ventricular function was found. Thus, the postoperative improvement in the exercise capacity both in mitral stenosis and in aortic regurgitation is associated with an improvement in right-ventricular function and a decrease in left-atrial size.