Normal M-mode echocardiographic values were analyzed from premature and term newborn infants with a body weight of 500 to 4000 g (n = 105) as well as from children ranging in age from one to 15 years with a body surface area of 0.4 to 1.6 m2 (n = 80). Standardized positioning of the recorded M-mode beam was assured through 2-dimensional echocardiographic monitoring. The maximal diastolic dimension of the right and left ventricles as well as the diastolic dimension of the aorta and the maximal dimension of the left atrium were assessed. The normal values were found to demonstrate linear relationships with weights up to 4000 g and, for larger children, with body surface areas from 0.4 to 1.6 m2. A review of values reported in the literature, of which there is a scarcity in the premature age, showed good general agreement of the mea values. The normal ranges, however, differed markedly, in some cases to such an extent that interpretation of individual echocardiographic findings would be rendered quite problematic. The explanation for these discrepancies appears most likely based on inconsistencies in spatial orientation and, subsequently, inaccurate measurements. It is apparent that establishment of normal values in a large patient population prerequisites guidelines for standardization which should include monitoring of the M-mode beam with two-dimensional echocardiography as well as use of tracings of superior quality only.