Discovering trends and regional differences in the prevalence of overweight and obesity in children and adolescents is an important task, bearing in mind the high public health relevance of this widespread health risk. It is all the more important to ensure a common methodological basis for the calculations underlying statements about trends and comparisons. Using the data of the German Health Examination Survey for Children and Adolescents (KiGGS), the potential effects of two methodological sources of error when calculating overweight and obesity prevalence at school entry are presented. Finally, simple recommendations for avoiding these errors are given.