OBJECTIVE: This article aimed to identify the influence of high serum homocysteine (Hcy) and cysteine (Cys) levels, alone or in conjunction, on changes in anthropometric parameters in children and adolescents over a 12-month follow-up period. METHODS: This is a cohort study involving 483 boys and girls 7-15 years of age. The outcome variables were body mass index (BMI)-for-age and waist circumference (WC) and the principal exposure variables were serum Hcy and Cys levels, alone or in conjunction. A generalized estimating equation (GEE) approach was used to identify longitudinal changes in the outcome variables. RESULTS: Irrespective of age, sex, socioeconomic conditions, dietary intake, or the practice of physical activity, the children and adolescents in the fifth quintile of distribution of Hcy levels had a z-score increase of 0.50 (p < 0.01) and a 3.62 cm increase (p < 0.01) in mean BMI-for-age and WC, respectively, over the period of the study. In individuals with Cys values above the fifth quintile, a z-score increase of 0.59 (p < 0.01) and a 5 cm increase (p < 0.01) were found in BMI-for-age and WC, respectively. When serum Hcy and Cys levels were both above the fifth quintile of distribution, a z-score increase of 0.87 (p < 0.01) and a 6.57 cm increase (p < 0.01) were found in mean BMI-for-age and WC, respectively, over the 12-month follow-up. CONCLUSION: High serum Hcy and Cys levels contributed to an increase in BMI-for-age and WC in children and adolescents over a 12-month follow-up period, with these increases being even greater when these 2 biochemical parameters were simultaneously high.