Rationale Nitric oxide (NO) produced in the lung is an important mediator of normal lung development, vascular smooth muscle relaxation, and ventilation perfusion matching. NO is synthesized from arginine by the action of NO-synthase (NOS). Asymmetric dimethylarginine (ADMA), an endogenous derivate of arginine, inhibits NOS and is thereby a determinant of NO synthesis. We compared ADMA and arginine levels in preterm infants requiring mechanical ventilation with preterm infants who did not require mechanical ventilation and determined the relation between ADMA and the length of mechanical ventilation in these infants. Methods Thirty preterminfants, mean (SD) gestational age 29.3 (1.7) weeks and birth weight 1,340 (350) gram, of the Neonatal Intensive Care Unit of the VU University Medical Center were included. ADMA and argininewere measured in umbilical cord blood and the length of mechanical ventilation (days) was registered. Results Gestational age and birth weight were significantly smaller in infants requiring mechanical ventilation, butwere not significantly correlated with plasma ADMA concentration after birth. Plasma ADMA concentrations were significantly higher in infants who required mechanical ventilation than in infants who did not require mechanical ventilation (1.53 ± 0.23 and 1.37 ± 0.14 μmol/L, respectively; P = 0.036). ADMA concentration was significantly related to length of mechanical ventilation (B = 3.4; 95% CI: 1.1-5.6; P = 0.006), also after adjustment for gestational age (B = 2.3; 95% CI: 0.4-4.2; P = 0.024). Conclusions Preterm infants who require mechanical ventilation have increased ADMA levels compared to non-ventilated preterm infants. ADMA levels at birth are related to the length of mechanical ventilation. An increased ADMA concentration could reduce NO synthesis, which could lead to insufficient gas exchange and, consequently, a longer period of mechanical ventilation. © 2008 Wiley-Liss, Inc.