We evaluated 30 patients with dilated cardiomyopathy (New York Heart Association functional Class II or III with medical treatment) to assess the effect of mild mitral regurgitation (MR) on exercise capacity in patients with congestive heart failure. They were classified into two groups based on results of left ventriculography: MR present (n = 10) and MR absent (n = 20). The severity of the MR by left ventriculography was grade I (mild) in all patients with MR. Steady-state hemodynamic data and angiographic data did not differ significantly between the two groups. Heart rate and systolic blood pressure at rest and in response to symptom-limited exercise testing did not differ between the groups. However, the peak work load was significantly lower in the group with MR than that in the group without MR (101 +/- 32 vs. 142 +/- 29 W, respectively; p < 0.005). Peak oxygen uptake and peak oxygen pulse were also significantly lower in the group with MR than in that without MR (peak oxygen uptake: 18 +/- 23 +/- 5 ml/min/kg; p < 0.05, peak oxygen pulse: 6.6 +/- 2.6 vs. 9.5 +/- 2.7 ml/min/beat: p < 0.01, respectively). Thus, mild MR had a detrimental effect on the exercise capacity in patients with dilated cardiomyopathy.