PURPOSE: Arterial blood gas analysis is frequently performed in neonatal intensive care unit (NICU) to evaluate ventilation and the metabolic state of critically ill infants. In occasions when umbilical arterial catheterization is not available, frequent arterial puncture is mandatory. This requires some technical skill and may occasionally have side effects. So we studied the validity of capillary blood gas analysis which can be performed conveniently compared with arterial blood. METHODS: Twenty-four neonates admitted to NICU during April to Aug. 2001 were studied. They were more than two weeks old without indwelling arterial catheters. Thirty-six times, simultaneous arterial, and capillary blood gases were drawn by puncture and the pH, pCO2 and pO2 of each sample was measured. Blood pressure and body temperature was checked before sampling to rule out impaired peripheral circulation. Capillary blood was collected from warmed heels. RESULTS: There was a strong correlation between capillary and arterial pH(r=0.91, P<0.05). The absolute value of the difference between arterial and capillary pH was less than 0.05. Also capillary pCO2 showed correlation with arterial pCO2(r=0.77, P<0.05). Despite a statistically significant correlation between capillary and arterial pO2(r=0.68, P<0.05), the absolute value of the difference was more than 10 mmHg in 92% of cases. CONCLUSION: Capillary blood gases accurately reflected arterial pH and pCO2 and showed a relative correlation with pO2. Capillary blood gas analysis can be a useful alternative to arterial blood when continuation of the umbilical arterial catheter is no longer available.