Overweight and obesity are major public health issues. Childhood obesity often persists throughout adulthood. Recently a higher prevalence of obesity in adults whose mothers smoked during pregnancy was reported. The aim of this study was to assess whether this association is also detectable in pre-school children in a different setting and to identify the critical period for intrauterine exposure to inhaled smoke products in pregnancy. We analysed questionnaire data on early feeding and lifestyle factors of 8,765 German children aged 5.00 to 6.99 years. Obesity was defined as a body mass index >97th percentile. The prevalence estimates for obesity were: mother never smoked 2.8% (95% CI 2.4%-3.2%), smoked after pregnancy only 1.6% (95%CI 0.4%-4.1%), smoked throughout pregnancy 6.2% (95% CI 4.5%-8.3%), smoked before pregnancy, but not throughout 4.5% (95%CI 3.6%-5.7%). These associations could not be explained by confounding due to a number of constitutional, sociodemographic and lifestyle factors. The unadjusted/adjusted odds ratios were: smoked during pregnancy: 2.32 (95% CI 1.63%-3.30%)/1.92 (95% CI 1.29%-2.86%); smoked before, but not throughout pregnancy: 1.67 (95%CI 1.26%-2.22%)/1.74 (95%CI 1.29%-2.34%). CONCLUSION: the association of maternal smoking in pregnancy and obesity was also detectable in children at school entry. Since smoking after pregnancy was not associated with childhood obesity, intrauterine exposure rather than family lifestyle factors associated with smoking appears to be instrumental. There appears to be a role for early intrauterine exposure.