The aim of this study was to determine changes in the prevalence of undernutrition, overweight and obesity in Argentine children and adolescents over the last two decades, and analyze their relationship with family socioeconomic conditions. Methods: We compared data in two cross-sectional studies (C) of schoolchildren (n: 4971) aged 4 to 13 years from three provinces: Buenos Aires, Mendoza and Misiones. C1 was carried out between 2005 and 2009 and C2 between 2010 and 2019. Anthropometric (body weight, height, triceps and subscapular skinfolds) and socioeconomic data of both studies were obtained in the same schools by the same investigators, using a similar methodology. The prevalence of nutritional status and the frequency of family socioeconomic conditions were calculated by province and C. Data were analyzed by chi square test, categorical principal component analysis and generalized linear model. Results: The prevalence of undernutrition decreased in C2 in Buenos Aires and Misiones and remained constant in Mendoza. At the same time, the prevalence of overweight and obesity increased in the three provinces, around 13% in Buenos Aires and 9% in Mendoza and Misiones. Similarly, fat mass increased in C2. Undernutrition was associated with inadequate family socioeconomic conditions only in C1.Conclusions: This 14-year follow-up study shows a decreasing trend in the prevalence of undernutrition and an increasing trend in the prevalence of overweight, obesity, and fat mass in the child and adolescent population of Argentina. Also, a marked deterioration in family socioeconomic conditions was found, but its relationship with the changes in nutritional status remains to be elucidated.