Background: Cigarette smoke is a major source of free radicals and oxidative stress. With a significant proportion of women still smoking during pregnancy, this common and avoidable exposure has the potential to influence infant oxidative status, which is implicated in the increased propensity for airway inflammation and asthma. The aim of this study was to examine the effects of maternal smoking on markers of infant oxidative stress.Methods: The level of oxidative stress (using urinary F-2-isoprostanes as a marker of lipid peroxidation) was compared in infants of smokers (n = 33) and non-smokers (n = 54) at 3 months of age. These groups were balanced for maternal atopy and socioeconomic status. Infant urinary cotinine levels were also measured as an indicator of early postnatal cigarette smoke exposure.Results: Maternal smoking was associated with significantly higher infant cotinine levels, despite the fact that most smoking mothers (83.8%) claimed not to smoke near their baby. Maternal smoking was associated with significantly higher markers of oxidative stress (F-2-isoprostane) at 3 months of age. There was also a positive correlation between urinary F-2-isoprostanes and infant urinary cotinine levels.Conclusions: Although this study does not separate the prenatal and postnatal effects of smoking, these findings indicate that environmental tobacco smoke in the early postnatal period adversely affects pro-oxidative/ antioxidative status within weeks of life in very early infancy.