The aim of this study was to examine the changes in body composition (fatness and muscle and bone mass) and BMI in three cohorts of boys living in Krakow, Poland, over the last 30 years, and to assess how overweight and obesity frequencies have developed over the period. The anthropological survey was conducted in 2010, and comprised 1863 boys from Krakow aged 3–18 years. The analysis compared the latest measurements of selected skinfold thicknesses, BMI, total body muscle mass and bone mass with the results of surveys from 1983 (N=1934) and 2000 (N=2323). The frequency of overweight and obesity was assessed for the last survey series, and their trends determined for the years 1983–2000 and 2000–2010. The analysis of fat tissue distribution showed that the boys from the 2010 cohort, as compared with those examined in 2000, had a higher trunk adiposity, as manifested by greater thickness of subscapular and suprailiac skinfold thicknesses, and boasted a larger muscle mass and lower bone mass. The survey results also showed that, from 1983 to 2000, the percentage of overweight boys increased slightly (from 11.69% to 12.48%), whereas the frequency of obesity did not change. Over the last decade, the percentage of overweight people has increased to 15.94%, and the frequency of obesity has doubled, equalling 4.94% in 2010. The last decade has seen a minor trend in boys towards increased trunk adiposity, muscle mass, BMI and prevalence of overweight and obesity, and a slight decrease in bone mass.