The objective of this study was to examine a possible association between maternal smoking in pregnancy and childhood overweight. From a population-based cohort of 5722 women from Trondheim, Bergen (Norway) and Uppsala (Sweden) enrolled in early pregnancy during 1986-92, a random sample of 482 women was selected for participation. They were followed up throughout pregnancy, and their children from birth until 5 years of age. Data on maternal smoking and diet, socio-economic determinants and breast feeding were recorded prospectively. During pregnancy and childhood, anthropometric measures were also recorded. Maternal smoking status was based on reported number of cigarettes smoked in week 17 of pregnancy. Child overweight was defined by body mass index (BMI) and sum of skinfold thickness (SFT) >or= 85th percentile at 5 years of age. Children of mothers who smoked in pregnancy had increased risk of overweight at 5 years of age (RR 2.5, 95% CI 1.5, 4.2 for BMI; and RR 1.8, 95% CI 1.1, 3.0 for SFT). Adjusting for maternal diet, breast feeding, maternal obesity and socio-economic status did not suggest confounding. However, adjustment for birthweight increased the observed risk. A linear increase in BMI and SFT was observed with increasing number of cigarettes smoked. In conclusion, smoking during pregnancy may be a risk factor for development of childhood overweight. This study may support the hypothesis of 'fetal origin of adult disease', but the risk of overweight associated with smoking during pregnancy was independent of intrauterine growth retardation, and may thus be attributed to specific effects of cigarette smoke.