Aims: The criterions for the timing of surgical intervention in children with rheumatic mitral or aortic valvar regurgitation are not defined. I hypothesized that, in children with chronic mitral or aortic regurgitation, an index for decompensation could be created by using the ratio of the diastolic left ventricular wall thickness to the radius, and that such an index could prove useful in determining the optimal time for surgical intervention. Methods: The left ventricular echocardiograms were obtained at the tips of the leaflets of the mitral valve by M-mode echocardiography. The diastolic septal wall thickness was measured between the right and left ventricular endocardial layers, and the posterior wall thickness between the endocardium and the interphase between the epicardium and the myocardium. The left ventricular diastolic dimension was then measured, between the posterior and septal wall endocardial layers, and systolic dimension as the smallest distance detected between these layers. All diastolic measurements were made at the time of the R wave of electrocardiogram, using the leading edge technique. The ratio of wall thickness was measured using the mean of septal and posterior wall thicknesses divided by half the diastolic dimensions, the normalized thickness of the wall previously referred to as the h/r ratio and relative mural thickness. Results: The ratio of wall thickness to left ventricular radius, and its relation to systolic left ventricular pressure or systolic blood pressure, was found to be linear in 89 normal school children, and 39 children with aortic stenosis. For future predictions, I calculated the 95th percentile limits and the 95th percentile confidence bands for this relation. Using the same data, it proved possible to calculate ratios of wall thickness for various ranges of either systolic blood pressure or left ventricular peak pressure. By using the normal limits of 0.356 plus or minus 0.0316 of the ratio, appropriate for the systolic blood pressure of children with mitral regurgitation, I determined the adequacy of the ratio of wall thickness. Of the children, 51 were in ventricular failure, and these had an inadequate ratio, below two standard deviation. Of the others, 21 had an inadequate ratio to within minus one to minus two standard deviations, and 12 of these were asymptomatic, 8 were symptomatic, but only one was in ventricular failure. For 18 children with aortic regurgitation, using the same limits, one child was within 1 standard deviation and was asymptomatic, 8 fell within minus 1 to minus 2 standard deviations and 2 of these were symptomatic, 5 were in ventricular failure, and 1 was asymptomatic, while the other 9 had ratios falling less than minus 2 standard deviations, and all were in ventricular failure. Conclusion: I conclude that the index of normalized wall thickness defined as the ratio of the left ventricular wall thickness to its radius is adequate, and within normal limits, when there is compensated volume overload, but is inadequate and below normal limits when the volume overloaded left ventricle becomes decompensated. My data suggests that the persistently decreasing ratio of wall thickness below the limits of normality serves as an indicator of ventricular decompensation, and thus can be used as a new criterion for determining the optimal time for surgical intervention.