Objective: This study was designed for quantification of mitral regurgitation by echocardiographic measurements such as regurgitant volume (RV), regurgitant fraction (RF) and effective regurgitant orifice area (EROA), and to assess the effect of angiotensin converting enzyme inhibitor (ACEI) therapy on these measurements. Materials and Methods: Patients with rheumatic mitral insufficiency were divided into two groups: Study group (SG)-10 females, 2 males aged 10-18years, body surface area 1.49±0.05m2, receiving digoxin therapy for at least one year and Control group (CG)-8 females, 4 males, aged 8-17years, body surface area 1.38±0.07m2, with no treatment. Patients in the two groups had no symptoms of cardiac failure. Angiotensin converting enzyme inhibitor therapy was given to SG patients on admission. Echocardiographic examinations were applied on admission and at the 20th day of therapy with ACEI and digoxin. Results: Study group’s left ventricular end-diastolic volume (108.03±41.21 ml/m2), mitral stroke volume (510.37±321.58 ml/m2) and regurgitant volume (423.48±305.00 ml/m2) were significantly higher (p<0.05) on admission than in the CG (81.98±21.53 ml/m2, 315.34±207.38mL/m2 and 245.77±179.84mlm2, respectively). Aortic stroke volume at the 20th day of therapy was significantly higher in SG than in the CG. Therapy with ACEI decreased significantly SG’s left ventricular diastolic volume. Conclusion: Angiotensin converting enzyme inhibitors should be started at an early stage of mitral regurgitation. The effective regurgitant orifice area is a feasible and easy method for the outpatient follow-up of mitral regurgitation.