The long-term impact of cesarean delivery (CD) on the health of the offspring is being explored methodically. We sought to investigate the effect of birth by (a) prelabor and (b) during-labor CD on the risk of early-onset (≤3 years) acute lymphoblastic leukemia (ALL), specifically of its prevailing precursor B (B-ALL) subtype. A total of 1099 incident cases of ALL (957 B-ALL), 131 of acute myeloid leukemia (AML), and their 1 : 1 age-matched and sex-matched controls, derived from the Nationwide Registry for Childhood Hematological Malignancies (1996-2013), were analyzed using multivariate regression models. A null association was found between prelabor and/or during labor CD and either ALL (B-ALL) or AML in the 0-14 age range. By contrast, birth by CD increased significantly the risk of early-onset ALL [odds ratioCD (ORCD)=1.57, 95% confidence interval (CI): 1.10-2.24] mainly on account of prelabor CD (ORprelaborCD=1.66, 95% CI: 1.13-2.43). The respective figures were even higher for the early-onset precursor B-ALL (ORCD=1.66, 95% CI: 1.15-2.40 and ORprelaborCD=1.79, 95% CI: 1.21-2.66), whereas no association emerged for early-onset AML. Prelabor CD, which deprives exposure of the fetus/infant to the presumably beneficial effect of stress hormones released in both vaginal labor and during labor CD, was associated exclusively with an increased risk of early-onset ALL, particularly the precursor B-ALL subtype. If confirmed, these adverse long-term outcomes of CD may point to re-evaluation of prelabor CD practices and prompt scientific discussion on the best ways to simulate the effects of vaginal delivery, such as a precesarean induction of labor.