Objective. To determine the cut-off values for the body-mass index (BMI) for underweight and serious underweight in children up to 18 years of age based on the Dutch growth standards of 1980, and to determine the prevalence of underweight and serious underweight in the 1997 Dutch growth-study figures. Design. Secondary data analysis. Method. Cut-off values for underweight were determined by first calculating the percentage of 18-years old with a BMI < 18.5 kg/m2 in 1980, followed by a calculation of the BMI at younger ages under which the same percentage fell. Serious underweight in 18-year olds was defined as a BMI < 17.0 kg/m2. Results. Between 1980 and 1997 the prevalence of serious underweight increased in children aged between 2 and 6 years from 1.5% to 3.6% and from 2.9% to 3.3%, in boys and girls respectively. In older children (7-18 years) the prevalence decreased from 1.5% to 1.4% for boys and from 2.9% to 1.7% for girls. Conclusion. Not only did the median weight of the population increase between 1980 and 1997, but there was also a rise in the variation of the weights at any given age. The latter could reflect more marked health differences.