OBJECTIVE: To determine whether changes in mean BMI and the prevalence of obesity in a total adult population during a short (11-year) period were associated with changes in the prevalence of diabetes. RESEARCH DESIGN AND METHODS: This study involved cross-sectional surveys of all inhabitants aged > or = 20 years of the county of Nord-Trøndelag from 1984 to 1986 (n = 85,100) and from 1995 to 1997 (n = 92,434). Attendance rates were 88.1 and 71.3%, respectively, and 90.0% in an additional survey of people aged 13-19 years from 1995 to 1997 (n = 9,593). Main outcome measures were age-specific mean BMI for the diabetic and nondiabetic subgroups and the prevalence of obesity and diabetes. For comparison, mean BMIs from 18 of 19 Norwegian counties for the group aged 40-42 years were examined. RESULTS: Mean BMI increased from 27.2 to 29.0 kg/m2 in the diabetic population and from 25.1 to 26.3 kg/m2 in the nondiabetic population. The BMI distribution curve shifted to the right, but homogeneity was also reduced. A comparison with other Norwegian counties indicated that this increase occurred during the last 6 years between the surveys. The prevalence of obesity (BMI > or = 30 kg/m2) increased from 7.5 to 14% in nondiabetic men and from 13 to 18% in nondiabetic women. The increase was particularly great in men aged < 60 years and in women aged < 50 years. The overall prevalence of known diabetes increased between the two surveys (from 2.9 to 3.2%) but only in men. The largest increase was observed in the corresponding younger sex and age-groups. CONCLUSIONS: A substantial increase in mean BMI and the prevalence of obesity occurred in the younger age-groups at the same time as an increase in the prevalence of diabetes. A greater increase in diabetes prevalence in this ethnically stable Western European population may follow if effective primary preventive strategies are not undertaken.