Introduction: Our aim was to establish body mass index (weight/height² [kg/m²], BMI) reference values for 0- to 45-year-old Danes. 

Material and methods: A national sample that was used to generate the current Danish height and weight reference (29,106 measurements in 1965-1977; age 0-21 years; sample 1). Four samples from Copenhagen (3391 measurements in 1981-1985; age 7-45 years; samples II and III and 2608 measurements in 1991-1994; age 6-45 years; samples IV and V). With the LMS method for smoothing the curves, Danish BMI reference values by age and sex were constructed from samples I and II. BMI values from two recently examined, but smaller, Danish cohorts (samples IV and V) were compared with the reference values to assess the secular trend in BMI. 

Results: The Danish BMI reference values are presented graphically. Graphs can be downloaded from http://www.paediatri.dk. In the more recently examined samples, the mean BMI was significantly increased in children and adolescents (SD score 0.19, P=0.0007), but unchanged in young adults (SD score 0.01, P=0.49). 

Discussion: For clinical purposes, we recommend comparison of the BMI of Danish children and adolescents with the present Danish reference values and new internationally based cut-off values. The BMI of Danish adults should be compared with the WHO cut-off values. Cut-off values or national or internationally used BMI reference values can be used for research, depending on the purpose.