Background: It has been proposed that childhood vaccinations protect against acute lymphoblastic leukaemia (ALL) in children by modulation of future responses to common infections in childhood. However, the available studies provide inconsistent findings, and population-based cohort studies with longitudinal information on vaccinations are lacking.Methods: In a register-based cohort of all children born in Denmark from 1 January 1990 to 31 December 2008, followed up until age 15 years or 31 December 2009 (n=1 225 404), we evaluated exposure to childhood vaccination and risk of childhood ALL, including information on ALL subtypes. Using Cox regression, we estimated hazard ratios (HRs) comparing vaccinated with unvaccinated children.Results: Childhood ALL was diagnosed in 490 children during 10 829 194 person-years of follow-up. Neither the total number of vaccine doses received nor exposure to each vaccination given in childhood was associated with altered risk of ALL, including the following: (i) Haemophilus influenzae type b [HR, 1.04; 95% confidence interval (CI), 0.68-1.61]; ii) measles, mumps and rubella (HR, 1.01; 95% CI, 0.76-1.34); iii) whole-cell pertussis (HR, 1.10; 95% CI, 0.51-2.39); and iv) diphtheria, tetanus and inactivated polio (HR, 1.14; 95% CI, 0.42-3.13). Analyses conducted according to ALL subtypes defined by immunopheno- and karyotypes showed no association with childhood vaccination.Conclusions: This nationwide cohort study provides no support of the proposed protective effect of childhood vaccination against childhood ALL.