This study was designed to evaluate left ventricle dimensions in preterm infants during the first month of life, in order to define reference values and their correlation with gestational age, birth weight, gender and baseline. Thirty-five infants, gestational age 25-29 (mean 27.9 +/- 1.4) weeks, birth weight 750-1249 (mean 965 +/- 206) g, were measured using echocardiography on days 3, 7, 14, 21 and 28 of life. The following dimensions were measured: end-systolic and end-diastolic interventricular septum thickness, end-systolic and end-diastolic left ventricle posterior wall thickness, end-diastolic and end-systolic left ventricle diameter. A progressive and significant increase of all the left ventricle measurements was observed during the first month of life. Left ventricle dimensions at the first scan (Day 3) correlated with birth weight but not with gestational age and gender. The degree of the increase observed during the first month of life was inversely related to the baseline, suggesting that the smaller the left ventricle is at birth, the higher is its postnatal increase toward dimensions similar to those of term infants. Our study gives reference data about left ventricle dimensions of preterm infants during the first month of life and is helpful when making a diagnosis of left ventricular hypertrophy in these subjects.