Troponin I (TnI), an inhibitory protein complex located on the actin filament of cardiac muscle, has become a specific marker of myocardial damage. Troponin has been studied in a wide range of clinical settings. However, many questions are still unanswered, especially in preterm neonates with the most common pathology at birth, such as idiopathic respiratory distress syndrome (IRDS). The aim of this study was to establish a reference range for cardiac TnI for healthy preterm infants and serum levels in sick preterm infants with IRDS. Echocardiography was performed and TnI serum levels were measured at a median age of 62 hours of life in three groups of healthy preterm infants (n = 10), and ventilated infants with moderate (n = 15) and severe IRDS (n = 15). Ventilated infants with idiopathic moderate IRDS had significantly different cardiac parameters (R/L ejection fraction, R/L stroke volume, R/L cardiac output; p < 0.05) and significantly higher cardiac TnI levels than healthy infants (0.037 versus 0.01 microg/mL; p < 0.05). Furthermore, infants with severe IRDS had higher TnI concentrations than infants with moderate IRDS (0.26 versus 0.037 microg/mL; p < 0.05). The results of this study show that increased TnI serum levels in sick preterm infants with IRDS are explained by myocardial injury or dysfunction due to impaired arterial oxygenation or reduction in cardiac output during treatment with mechanical ventilation. These results suggest that cardiac TnI may be a useful, specific marker for myocardial damage in preterm neonates with IRDS.