A global epidemic in overweight and obesity in children has been postulated. There is a lack of consensus over definitions however, and national standards of BMI centiles have been published in various countries. This has made results on the prevalence of overweight difficult to interpret. Internationally based cut-off points have now been published, and a British study reports trends in overweight and obesity in children, as defined by these. The aim of this study was to compare the BMI of a sample of Danish children attending school in 1986/1987-1996/1997 to that of Danish schoolchildren in 1971/1972. The school health officers in 23 municipalities representing a "mini-Denmark" collected from the school health records 3-4 routine measurements of height and weight in children attending school in 1986/1987-1996/1997. From these measurements, the children's BMIs were calculated and BMI centiles were constructed. The data covered a total of 11,167 measurements of 3371 children. The children in our study had higher BMI values than had children in 1971/1972, both on average and for the highest centiles. The average BMI values were significantly higher from the age of 8 years in boys and 7 years in girls. The prevalence in overweight and obesity has increased. For 14-16-year-old boys, the average BMI was related to the educational level of the mother (negatively). The highest BMI centiles had especially increased. The proportion of overweight and obese 14-16-year-olds according to internationally based cut-off points had increased from 5.1% to 1.5% for boys and from 6.2% to 15.6% for girls since 1971/72. This trend calls for preventive efforts.