Background: Neonatal sepsis is a major problem in newborn nurseries because of the difficulty in early diagnosis and because of the high morbidity and mortality. The objective of the present study was to investigate whether urinary nitric oxide (NO) levels could be useful for the diagnosis of infected newborns. Methods: Newborns with suspected infection according to previously defined criteria between ages of 1-7 days and 8-30 days were included as the study groups (p) to be compared with age-matched healthy controls (c). Urine NO levels were assayed by Sievers NOA based on chemiluminescence and expressed as corrected for urine creatinine. Results: 20 newborns with suspected infection at 1-7 days of age (group 1p) were compared with 45 healthy age-matched newborns (group 1c). 16 newborns with suspected infection at 8-30 days of age (group 2p) were compared with 15 healthy age-matched newborns (group 2c). The groups were similar with regard to birth weight and gestational age; however, the urinary NO levels in newborns with suspected infection at 1-7 days of age (80.25 +/- 60.68 mu mol/mg creatinine) were higher than in healthy newborns (25.45 +/- 19.35 mu mol/mg creatinine). Similarly, newborns with suspected infection at 8-30 days of age had higher urinary NO levels (81.78 +/- 40.43 mu mol/mg creatinine) than age-matched controls (36.99 +/- 24.58 mu mol/mg creatinine; p < 0.05). The sensitivity of urinary NO levels to detect infection was 50% in both age groups, and the specificity was 95% for 1-7 days of age and 93% for 8-30 days of age. Groups Ip and 2p were similar with regard to NO production. Altogether 12 patients had culture-proven sepsis, 11 patients had clinical sepsis, and 13 patients had other infections. The NO levels were similar in patients with culture-proven and clinical sepsis and higher than in patients with other infections. No difference was observed among NO levels of patients with gram-positive and gram-negative sepsis. Conclusions: Urinary NO levels which are quick and easy to measure are higher in infected newborns as compared with controls, and although the specificity is good, the sensitivity of the test is low, necessitating the use of another marker in addition to NO. Copyright (C) 2000 S. Karger AG, Basel.