Prevalence of overweight and obesity in children under 5 years can be affected by type of anthropometric indicator and selected threshold values. We assessed variation on estimates according to national and international regulations (NOM-031-SSA2-1999, NOM-008-SSA3-2010, GPC-SSA-025-08 and GPC-IMSS-029-08; WHO-2006, CDC-2000 and IOTF, respectively). Cross-sectional study in all the daycare centers (100 %) affiliated to Instituto Mexicano del Seguro Social-Nuevo León during June-December, 2010 (n = 11 141 children). Overweight and obesity were defined on the basis of each regulation, and total and stratified prevalences by age and sex were estimated. According to national regulation, overweight/obesity estimates varied from 12 to 22 %, and to international normativity, from 3 to 14 %. The highest prevalence was given by NOM-008-SSA3-2010 and GPC-SSA-025-08; and the lowest by WHO-2006. There were no differences by sex, but they existed by age; the highest discrepancy occurred in the 3.0-3.9 age group with 28 %. Health personnel and health policy makers should be aware of variation on estimates according to the definition employed. We recommend to standardize national regulation for pointing out overweight/obesity in preschoolers.