Background: The issue of whether 21% O2 is more effective than 100% O2 for resuscitation of newborn infants remains controversial. Objectives: We have updated the systematic review and meta-analysis including all studies reporting resuscitation of newborn infants with 21 or 100% O2. Methods: Randomized or quasi-randomized studies of depressed newborn infants resuscitated with 21 or 100% O2 with or without masking of treatment were considered for inclusion. The outcomes of interest included neonatal mortality and hypoxic ischemic encephalopathy. Results: Ten studies fulfilled the inclusion criteria. Of these, 6 studies were identified as being strictly randomized. In total, 1,082 infants were allocated to resuscitation with 21% O2 and 1,051 infants with 100% O2. The risk of neonatal mortality was reduced in the 21% O2 group compared to the 100% O2 group both in the analysis of all studies (typical RR 0.69, 95% CI 0.54, 0.88) and in the analysis of strictly randomized studies (typical RR 0.32, 95% CI 0.12, 0.84). A trend toward a decrease in the risk of hypoxic ischemic encephalopathy stage 2 and 3 was noted with resuscitation in 21% O2 in the analysis of all studies (typical RR 0.88, 95% CI 0.72, 1.08). Conclusions: There is a significant reduction in the risk of neonatal mortality and a trend towards a reduction in the risk of severe hypoxic ischemic encephalopathy in newborns resuscitated with 21% O2.