Background: The analysis presented in this paper aimed to identify trends in the prevalence of the two extremes of body fat disorders over a period of 20 years (1979-1999) and to form a comparative basis for further studies of physical development of the youth of the Pomerania region.Material/Methods: The study population consisted of 23,854 boys and girls aged 7 to 18 years attending schools in rural and urban areas in the Pomerania administrative region. An overweight and underweight population was identified based on the body mass index (BMI) according to WHO, CDC and the Mother and Child Institute recommendations. The following threshold values were adopted: BMI≤ 5th centile for underweight, BMI between 5th and 15th centile for slimness, between 85th and 95th centile for overweight, and BMI≥ 95th centile for obesity).Results: The highest percentage of boys with a BMI between the 85th and 95th centile was seen in 1999, with figures of 8.7% among younger boys and 7.9% among older boys. This represented respective increases of 3.2 and 2.2 percentage points over 1979 data. The proportion of overweight boys (7 - 18 years) increased from 5.6% in 1979 to 7.4% in 1989 and 8.4% in 1999. The differences between these percentages (among both younger and older boys) in the study period (1979-1999) were statistically significant. Comparing 1989 and 1999, statistically significant differences were only identified among younger boys, and between 1979 and 1989 differences were statistically significant among older boys and for the entire study population.Conclusions: The 20-year period was marked by increasing prevalence of overweight among the boys and girls. Awareness of the consequences of developmental dysharmonies in the form of over- and underweight demands that these trends be further monitored.