Although most Doppler-derived left ventricular diastolic function parameters undergo permanent changes during growth and maturation, isovolumic relaxation time (IVRT) normalized to heart rate has a constant value from the fourth month of life through adolescence. No reference values exist for IVRT in babies less than 3 months of age. However, this is the period when the most profound maturational changes in diastolic ventricular function are observed. To fill this gap, we conducted a survey with echocardiographic examinations concentrating primarily on the measurement of diastolic time intervals in 75 healthy term neonates and infants of up to 3 months of age (Table). These infants were either term newborns during their stay in the hospital after birth or scheduled patients referred to our outpatient department for the clearing of innocent heart murmurs. All patients had an uneventful history and physical examination, normal electrocardiogram findings, blood pressure within normal limits, and underwent a complete echocardiographic examination of the heart and the great vessels. We used 7.5to 5-MHz transducers on commercially available systems (77020AC, Hewlett Packard, and Sonos 1500, Hewlett Packard Medical Systems Inc, Andover, Mass). We derived IVRT and early filling acceleration time exclusively from pulsed wave Doppler shift signals, following the method described in a previous article. Differences between age groups were verified by the Kruskal-Wallis signed rank test and interactions between variables were tested by 1-way analysis of variance. The observer reliability of IVRT measurements was calculated from 20 randomly selected data sets. Correlation coefficients were good with R 0.96 for intraobserver and R 0.92 for interobserver vari-