Several studies have described reduced plasma concentrations of arginine, the substrate for nitric oxide synthase (NOS) in infants with necrotizing enterocolitis (NEC). No information on the plasma concentrations of the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA) in patients with NEC is currently available. We investigated whether plasma concentrations of arginine, ADMA, and their ratio differ between premature infants with and without NEC, and between survivors and non-survivors within the NEC group. In a prospective case–control study, arginine and ADMA concentrations were measured in ten premature infants with NEC (median gestational age 193 d, birth weight 968 g), and ten matched control infants (median gestational age 201 d, birth weight 1102 g), who were admitted to the Neonatal Intensive Care Unit. In the premature infants with NEC, median arginine and ADMA concentrations (μmol/l), and the arginine:ADMA ratio were lower compared to the infants without NEC: 21·4 v. 55·9, P = 0·001; 0·59 v. 0·85, P = 0·009 and 36·6 v. 72·3, P = 0·023 respectively. In the NEC group, median arginine (μmol/l) and the arginine:ADMA ratio were lower in non-surviving infants than in surviving infants: 14·7 v. 33·8, P = 0·01 and 32·0 v. 47·5, P = 0·038 respectively. In premature infants with NEC not only the NOS substrate arginine, but also the endogenous NOS inhibitor ADMA and the arginine:ADMA ratio were lower than in infants without NEC. In addition, low arginine and arginine:ADMA were associated with mortality in infants with NEC. Overall, these data suggest that a diminished nitric oxide production may be involved in the pathophysiology of NEC, but this needs further investigation.