Background: Transient tachypnea of neonate (TTN) and respiratory distress syndrome (RDS) of the newborn are the most common cause of early respiratory distress in the immediate neonatal period. There is increasing evidence to support the role for the activation of the renin angiotensin system during acute lung injury. Objectives: The purpose of this study was to determine if there is a relationship between angiotensin-converting enzyme (ACE) I/D polymorphism, ACE activity and TTN and respiratory distress syndromes. Methods: Nineteen neonates with TTN, 20 neonates with RDS and 21 control infants are studied for ACE polymorphism and serum ACE activity. Results: Twenty six (43.3%) patients have DD polymorphism, 19 (31.7%) patients have ID polymorphism and 15 (25%) patients have II polymorphism. Serum ACE activity is 43.5 +/- 1.8 (40-46) U/L in DD, 31.5 +/- 2.3 (28-36) U/L in ID and 22.1 +/- 2.1(19-46) U/L in II patient. Conclusions: The study could not find any difference in DD alleles and ACE activity between control group and TTN group. ACE polymorphism was not different between RDS group and control group in this study.