Acute rheumatic isolated mitral regurgitation was followed up in 86 children for 730 patient-years. The grade of severity of regurgitation was classified according to degree of left ventricular enlargement, which was based on an original method of correction of cardiothoracic ratio on the chest x-ray, taking into consideration the diaphragmatic level, and electrocardiographic criteria for age. A combination of both quantitative estimations, after the regression of a detectable pericardial effusion reduced false-positive radiologic and false-negative electrocardiographic results. Linear regressions of percentage of yearly disappearance of MR were presented in relation to the grades of severity; the mean yearly rate of disappearance of 5.8% increased to 9.08% in mild, and decreased to 1.84% in severe regurgitation. The effect of other beneficial factors such as the shortness of the pretreatment interval and regularity of prophylaxis could be demonstrated by keeping the grade of severity of regurgitation constant. Thus, chances for recovery in reported years after onset, up to ten years, can readily be calculated from the presented regressions with regard to the appropriate subgroups.