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Introduction: Overweight in children is a serious public health problem. The use of different weight classification systems in research and clinical practice results in variable the estimate of prevalences of overweight, which complicates follow-up of this health problem in the population. The study compared three child body weight classification systems by estimating the prevalence of overweight established by each system. Method: In 2010, a study was conducted in 259 five-year-old children at the time of routine childhood vaccination. The children’s height and weight were measured. The prevalence of overweight was determined and compared to the International Obesity Task Force (IOTF), the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) criteria. Results: According to the IOTF, 16.6% of children of the study were overweight (obesity 3.1%). According to the CDC, 24.3% of children were overweight (obesity 9.1%) and according to WHO, the prevalence was 26.3% (obesity 6.2%). According to the IOTF criteria, obesity affected more girls than boys (2.7% vs. 0.4%), whereas similar proportions were observed with the other two systems. Conclusion: This study demonstrates that the prevalence of overweight in children varies considerably depending on the classification system used. These results support the need to consider the system used in clinical practice and in research when monitoring the course of the prevalence of this health problem.