Background: The date of acute lymphoblastic leukemia (ALL) diagnosis has been studied regarding potential etiologic roles with contrasting results and the issue remains controversial. We analyzed seasonality of diagnosis in a homogenous Hispanic cohort. Design/Method: Clinical files and electronic databases of consecutive children and adolescents with ALL in Northeast Mexico over 2004-2018 were scrutinized. Data included date of diagnosis, age, sex, and risk group. Patients were divided into children <16 and adolescents 16 to 20 years. Statistical analysis included Chi-square tests for heterogeneity and multiple Poisson regression using parametric harmonic modelling to detect monthly variation. Results: During the fifteen-year study period 394 consecutive patients with ALL were included. There were 323 (81.9%) children, and 71 (18.1%) adolescents. 176 (54.5%) children had high-risk disease. Heterogeneity across months of diagnosis was confirmed (P=0.038). A statistically significant monthly variation in the date of diagnosis documented by Poisson regression was found for the whole group, with a peak on March (P < 0.001). When analyzing by age group, cases of children with ALL predominated in March. Adolescents had a separate peak in July, with a 4.5 : 1 male to female ratio. Conclusions: Seasonality at month of diagnosis in children and adolescents with ALL was identified. Male adolescents had a high July peak suggesting a hormonal influence in ALL development in this age group.