Background:We investigated the association between supplemental folic acid in pregnancy and childhood cancer in a nation-wide study of 687 406 live births in Norway, 1999-2010, and 799 children diagnosed later with cancer.Methods:Adjusted hazard ratios (HRs) compared cancer risk in children by approximated periconceptional folic acid levels (folic acid tablets and multivitamins (0.6 mg), only folic acid (0.4 mg), only multivitamins (0.2 mg)) and cancer risk in unexposed.Results:Any folic acid levels were not associated with leukemia (e.g., high-level folic acid HR 1.25; 95% CI 0.89-1.76, P Trend 0.20), lymphoma (HR 0.96; 95% CI 0.42-2.21, P Trend 0.51), central nervous system tumours (HR 0.68; 95% CI 0.42-1.10, P Trend 0.32), neuroblastoma (HR 1.05; 95% CI 0.53-2.06, P Trend 0.85), Wilms' tumour (HR 1.16; 95% CI 0.52-2.58, P Trend 0.76), or soft-tissue tumours (HR 0.77; 95% CI 0.34-1.75, P Trend 0.90).Conclusions:Folic acid supplementation was not associated with risk of major childhood cancers.