Background: The objective of this study was to determine changes in the group size of girls and boys with proper BMI values and their peers with underweight, overweight and obesity in a twenty-year time span. Material/Methods: In 1985 and 1986,20,353 boys and girls were examined, and 20 years later, 10,705 students. Measurements of body height and body mass were taken, which enabled calculating values of the Body Mass Index (BMI). The statistical significance of differences between mean values obtained in 1986 and 2006 was verified with Student’s t test for independent variables. The statistical significance of differences between the number of girls and boys classified to each group in respect of the whole group surveyed in 1986 and 2006 was determined with the χ² test. Results: The greatest generation changes in the mass-to-height proportions were noted in the youngest children from the Podlaskie Province. It was higher by 1.66% in pupils with 2nd degree underweight, and higher by 4.79% and 1.29% in pupils with overweight and obesity, respectively, while it was lower by 10.11% among boys with a normal BMI value. In girls the percentage with normal BMI values was observed to decrease by 8.95%, whereas among girls with overweight – to increase by 4.85%. Conclusions: The results may suggest that a greater percentage of the young generation will be characterized by improper mass-to-height proportions.