Exercise capacity is frequently impaired in patients with mitral stenosis (MS) and sinus rhythm (SR). The resulting increased heart rate, which shortens the diastolic filling period, and the increased cardiac output lead to further elevations of left atrial pressure and subsequent pulmonary congestion. The effect of the beta-receptor blocking agent atenolol, 100 mg/day, was assessed in 13 patients with MS and SR. Exercise performance was assessed using a modified multistage Bruce protocol after 2 weeks of placebo and after 2 weeks therapy with atenolol in a single-blind, crossover, placebo-controlled, randomized study. Atenolol resulted in significant decreases in mean heart rates at rest and during exercise (p = 0.0015) and a significant increase in total exercise time (p = 0.0015). Maximal exercise capacity was also significantly improved (p = 0.0015). All patients were both objectively and subjectively improved by atenolol. Thus, beta-blockade with atenolol improves exercise capacity in patients with MS and SR and may be of benefit to most such patients. The improved effort tolerance is attributed to reduction of the exercise-associated sinus tachycardia by beta-blockade, allowing a longer diastolic filling period and better left atrial decompression.