BACKGROUND Both gestational and chronological age of the neonate may influence and impair the function of the delicate and immature myocardium. However, the transition from fetal to neonatal circulation in preterm neonates is poorly understood. AIMS This study aimed to compare left ventricular (LV) systolic and diastolic function between premature neonates at expected term and term neonates during the postnatal cardiovascular transitional period. METHODS Using echocardiography, we assessed systolic and diastolic function of the LV in 89 preterm neonates at week 40 of postconceptional age and 29 term neonates after closure of the patent ductus arteriosus (PDA) and on the 28th day of life. Based on M‑mode images, we measured myocardial thickness and fractional shortening (FS%). Using pulsed‑wave Doppler echocardiography, we estimated cardiac output, myocardial performance index (MPI), and LV diastolic function (E and A waves, E/A ratio). Systolic and diastolic function was also assessed by tissue Doppler imaging. RESULTS Compared with term neonates on the 28th day of life, preterm neonates had reduced myocardial thickness (P ≤0.04), FS% (P = 0.002), and cardiac output (P = 0.01). However, preterm neonates had a lower MPI than term neonates after PDA closure (P <0.001) and on the 28th day of life (P = 0.02). The E/A ratio and S' wave values were similar in preterm and term neonates (P >0.05). CONCLUSIONS Preterm neonates at 40 weeks of postconceptional age have preserved systolic and diastolic function of the LV.