To estimate the changes in the prevalence of underweight among girls and boys living in Kraków that occurred between 1983 and 2010.

The study was based on two cross-sectional surveys conducted in 1983 and 2010. The prevalence of underweight was estimated based on the International Obesity Task Force (IOTF) and Centers for Disease Control and Prevention (CDC) cut-off points.

The study was conducted in Kraków, Poland.

Children aged 3–18 years (n 5245).

Between 1983 and 2010 the prevalence of underweight decreased in both sexes. Using the IOTF criteria, the prevalence of underweight decreased from 10·5 to 10·3 % in girls and from 8·9 to 7·5 % in boys, but it was still higher in girls than in boys. According to the CDC criteria, this prevalence decreased from 5·1 to 4·4 % and from 5·9 to 4·6 %, respectively, and was slightly lower in girls. According to the IOTF criteria, underweight prevalence increased slightly during childhood in both sexes while according to the CDC criteria it decreased in boys and did not change in girls. Among juveniles and adolescents, it decreased regardless of the method used.

The frequency of underweight in the population of children and adolescents from Kraków changed slightly in 1983 and 2010 despite economic and social changes. Depending on the cut-off points used, not only the magnitude but also the direction of changes between series could be different. In order to aid global monitoring, the prevalence of underweight as well as overweight should always be evaluated by several methods.