The political and socio-economic transition initiated in Poland in the late 1980s is still continuing and has affected all social classes as well as all spheres of daily life of the people. The impact of this change on nutrition and health is examined here, by comparing the body mass index (BMI) of a 10% nationally representative sample of male conscripts aged 19 years (18.50-19.49), in 1986 (n = 26,396), 1995 (n = 22,612), and 2001 (n = 26,178). Mean BMI of young men did not change between 1986 and 1995, but then increased slightly from 22.0 to 22.3 between 1995 and 2001. There was also a significant and continuous increase in the variation of BMI in all social strata across the entire period 1986-2001. This effect is attributed to economic modernisation that has allowed increased diversity of life styles across occupational groups and between families. Sibship size was more important than socio-economic status in explaining variation in BMI after the political changes in 1990. The BMI-enhancing effect of small sibship size is attributed to nutritional intake, and to changes in food availability across the period under study.