Peak systolic left ventricular pressure was predicted in five infants with aortic stenosis by use of a wall stress constant, K. K was determined in 10 normal infants according to the formula K = P . Ds/Ws, where P = arterial pressure by Doppler, DS = end-systolic LV dimension, and WS = end-systolic wall thickness. Left ventricular peak systolic pressure was estimated in five infants using the formula P = K. Ws/Ds. Excellent correlation was obtained between measured left ventricular pressure and left ventricular-aortic pressure difference at cardiac catheterisation and echo estimates. Echocardiographic assessment of the severity of aortic stenosis may be applied to infants with good results.