The aim of this thesis is to compare effects of respiratory distress syndrome (RDS) on the myocardial function of newborn preterm infants and the later effects of chronic lung disease of prematurity on pulmonary artery stiffness in school age children. The first study in this thesis compared global and regional myocardial function in preterm infants with respiratory distress syndrome (RDS) with preterm and term-born controls (30 with RDS, 30 preterm control ≤34 weeks, 60 term control) using conventional and tissue Doppler echocardiography at birth, at term, one month, and one year of age. The second study compared the pulmonary artery stiffness, an early preclinical marker of pulmonary hypertension, in children (aged 8-12 years) who had chronic lung disease of prematurity (CLD) with preterm and term-born controls. Pulmonary artery pulse wave velocity (PA PWV) was assessed in 59 children: 13 with CLD, 21 preterm (≤ 32 weeks gestation) and 25 term controls) using velocity encoded MRI technique while breathing room air and after 20 minutes of breathing 12% oxygen. At birth, infants with RDS had lower pulmonary artery AT:ET (p < 0.001), long axis shortening (p < 0.01), RV systolic velocity (p < 0.001) and higher TR (p < 0.01) compared to preterm and term control groups. The preterm groups was also noted to have diastolic dysfunction (lower mitral E:A) at birth (p < 0.001). At term corrected age, pulmonary artery AT:ET was still lower in the RDS group but no differences detected in TR between the groups. There were no differences in all parameters measured between the groups at one month and one year. 2 PA PWV was similar in all three groups at baseline when assessed at school age. However, following hypoxic challenge, PA PWV in children who had CLD increased significantly compared to preterm (p=0.025) and term controls (p=0.042). The findings in this thesis suggest that infants with RDS had mildly elevated pulmonary arterial pressure as a result of milder respiratory disease with improvement in antenatal and neonatal care. The RV global dysfunction in infants with RDS resolved with resolution of the respiratory condition. Both preterm groups underwent postnatal maturation of myocardial function and caught up with the term control group by one month corrected age. At school age, children who had CLD displayed increased pulmonary vascular reactivity to hypoxia and are at greater risk of developing pulmonary hypertension earlier.