Fetal outcome in 77 uneventful pregnancies was examined and related to venous cord carboxyhaemoglobin (HbCO) levels. 30 women were smokers, 47 were non-smokers. Birth weight and birth weight centiles were found to be substantially reduced in children of mothers who smoked. HbCO levels were significantly elevated in venous cord blood of children of smokers compared with non-smokers. The role of fetal HbCO as a causal factor in reducing birth weight centiles of children of smoking mothers is discussed. It is concluded that carboxyhaemoglobin concentration in fetal venous cord blood did not account for fetal growth retardation in pregnant women who smoked.