OBJECTIVE: To establish an Inuit body mass index (BMI) norm from a healthy, not malnourished, pure Inuit population and to investigate the development of overweight in the Inuit in Greenland. DESIGN:Longitudinal study with 35 years follow-up on overweight among Inuit in Greenland. METHODS: The heights and weights of 97% of all inhabitants in Eastgreenland in 1963 (n=1852) were recovered recently and BMI calculated. We obtained similar data in 96% of the 50-69-year-old population in Eastgreenland in 1998 and in a random sample of 25% of individuals aged 50-69 years in the capital Nuuk (n=535). RESULTS: Overweight or obesity, as defined by the World Health Organization (WHO), was found in 30% of all men and 22% of all women in Eastgreenland in 1963, and in 31% of young Inuit hunters in 1963. Such high rates were incompatible with a hunter's way of living. Inuit-specific BMI norms from data on healthy Inuit aged 20-29 years in 1963 were computed: men, 20.2-27.9; women, 17.9-27.7. These differed from the WHO classification (P<0.001). Using the Inuit-specific BMI norm for the classification of 50-69-year-old Inuit in 1963 and 1998, the fraction of overweight men increased by over six times (4.0 to 25.6%; P<0.001), and overweight increased with Westernization (P=0.001). The fraction of overweight women by the Inuit BMI norm doubled from 1963 to 1998 (14.0 to 30.7%; P<0.001) while median BMI remained unaltered (P=0.22) because the fraction of slim women more than doubled (3.5 to 9.0%; P<0.001). CONCLUSION: A steep increase in the fraction of overweight Inuit men and women calls for intervention. Westernization predicted increased BMI. In women the increased number of obese people was accompanied by an increased fraction of slim people. This illustrates that transition can be modified and indicates that monitoring of populations in transition should observe gender differences. Finally, the historical data argue against the global applicability of the WHO delineation of normal BMI.