Objective To investigate the association between delivery by caesarean section and risk of childhood cancer.Design A population-based, follow-up study using register data from three countries.Setting Denmark, Sweden and Finland.Population Children born in Denmark (1973Denmark ( -2007, Sweden and Finland (randomly selected sample of 90%, 1987-2007; n = 7 029 843).Methods Exposure was delivery by caesarean section and the outcome was childhood cancer diagnosis. Follow-up started from birth and ended at the first of the following dates: cancer diagnosis, death, emigration, day before 15th birthday or end of follow-up. Cox regression was used to obtain hazard ratios. Main outcome measures Childhood cancer diagnosis.Results A total of 882 907 (12.6%) children were delivered by caesarean section. Of these, 30.3% were elective (n = 267 603), 35.9% unplanned (n = 316 536) and 33.8% had no information on planning (n = 298 768). Altogether, 11 181 children received a cancer diagnosis. No evidence of an increased risk of childhood cancer was found for children born by caesarean section (hazard ratio, 1.05; 95% confidence interval, 0.99, 1.11). No association was found for any major type of childhood cancer, or when split by the type of caesarean section (elective/unplanned). ConclusionThe evidence does not suggest that caesarean section is a risk factor for the overall risk of childhood cancer and possibly not for subtypes of childhood cancer either.