The body mass index (BMI) is widely used as an indicator of nutritional status of individuals and groups, changes in this measure reflecting changes in living conditions. The purpose of this paper is to present long-term BMI trends of schoolchildren from Jena/Eastern Germany over 125 years. The analysis is based on 10 anthropological investigations carried out between 1880 and 2005/2006 and belongs to the longest running, continuous investigation of schoolchildren in a single community worldwide. The average BMI of the 7- to 14-year-old probands generally increases between the first investigation in 1880 and the last study in 2005/2006 by 1.8 kg/m(2) in boys and by 2.1 kg/m(2) in girls. However, this long-term increase of the BMI does not occur continuously. The trend was interrupted for example by declining average BMI values between 1932 and 1944. In contrast, the time after German reunification is characterised by a marked increase in this measure. The changes in BMI of Jena schoolchildren are also analysed in relation to energy intake in different years. There is an association between changes in energy intake and changes in BMI, which weakens after 1985. This indicates that the recent dramatic increase in the prevalence of overweight and obesity is not only due to an increased energy intake but also reflects a more inactive lifestyle of children and adolescents. The continuing change in the BMI pattern of Jena children and adolescents is discussed in regard to programmes of prevention of overweight and obesity.