OBJECTIVES: To generate body mass index (weight/height(2) (kg/m(2)), BMI) reference values for 0 to 45-y-old Danes and compare these with published European reference values. SUBJECTS: A national sample used to generate the current Danish height and weight reference (29 106 measurements made 1965-1977; age 0-21 y; sample I), and four samples from Copenhagen (3391 measurements made 1981-1985; age 7-45 y; samples II-III and 2608 measurements made 1991-1994; age 6-45 y; samples IV-V). DATA ANALYSIS: Using the LMS method, Danish BMI reference values by age and sex were constructed from samples I and II. These were compared with BMI reference values from Sweden (age 6-16 (girls) or 6-19 y (boys)), Germany (6-19 y), UK (0-23 y), and France (0-87 y). 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: Overall, Danish BMI reference values (samples I and II) fitted best with French reference values and were systematically below UK, Swedish and German reference values. However, the BMI centiles of young adult Danish women were above French reference values and the BMI of Danes was substantially below French and UK reference values during the first months of life. The mean BMI Z-score of the recently examined samples was 0.24 (sample IV, P=0.0001) and 0.15 (sample V, P=0.0001) based on the French reference values and 0.19 (sample IV, P=0.0007) and 0.01 (sample V, P=0.49) based on the Danish reference values. CONCLUSION: For clinical purposes, we recommend comparing the BMI of Danish children and adolescents with the present Danish reference values and the new IOTF cut-off values and comparing the BMI of Danish adults with the WHO cut-off values. For research purposes, cut-off values and national or internationally used BMI reference values may be used, depending on the research questions.