Background:: Causes of most childhood hematopoietic neoplasms are unknown. Early age of occurrence suggests prenatal etiology. Positive associations have been reported between industrially produced trans-fatty acids (iTFAs) and risks of some cancers in adults. iTFAs are pro-inflammatory and adversely affect the beneficial effects of essential fatty acids, the latter is diminishing tumor growth. In 2004 Denmark legislated against the use of iTFA in foodstuffs. Using the entire population, we investigated if the changes in the legislation as a proxy to the reduced exposure to iTFA had affected the incidence of childhood hematopoietic neoplasms. Methods:: We used a Cox proportional hazard model to compare the hazard of childhood hematopoietic neoplasms among children born before and after the iTFA ban, as a proxy for fetal iTFA exposure. To take the potential secular trend in hematopoietic neoplasms into account, we modeled the variation in cancer risk across birth cohorts by a piecewise linear spline with a knot in 2004, which allowed a comparison of the hazard of childhood hematopoietic neoplasms between the time before and after the iTFA ban. Results:: Among children born in 1988–2008 in Denmark, 720 were diagnosed with hematopoietic neoplasms before the age of 7 years, corresponding to an overall incidence rate of 7.6 per 100 000 person years. The incidence rates increased by 2% per cohort in 1988–2004 (hazard ratio: 1.02 [1.01; 1.04]) and in 2004–2008 (hazard ratio: 1.02 [0.95; 1.11]). Conclusions:: No apparent benefit of the iTFA legislation in reducing childhood hematopoietic neoplasms was observed on population basis. Individual-level data are needed to investigate any possible associations between biomarkers of iTFA intake and risk of childhood hematopoietic neoplasms.