Objectives To determine the mean body mass index (BMI, kg/m²) and prevalence of low weight (BMI<20) and obesity (BMI≥30) in 3 population-based surveys, and to describe the longitudinal changes during 1994–2008 in mean BMI, and the prevalence of low weight and obesity.

Setting A population study in Tromsø, Norway.

Participants A total of 29 688 different participants in 1 or more of 3 surveys (1994–1995, 2001–2002 and 2007–2008). Longitudinal analyses comprised 9845 participants aged 25–69 in 1994 who participated in the 1994–1995 and 2007–2008 surveys and 4202 men and women who participated in all 3 surveys.

Outcome measures Mean age-specific and sex-specific BMI, prevalence of low weight and obesity, and changes in BMI and prevalence of low weight and obesity during 1994–2008, according to sex and birth cohort.

Results The age-adjusted (ages 30–84) prevalence of obesity increased from 9.8% and 11.8% in men and women, respectively, in 1994–1995 to 20.9% and 18.5%, respectively, in 2007–2008. The increase in mean age-adjusted BMI was stronger from 1994–1995 to 2001–2002 than from 2001–2002 to 2007–2008. Longitudinal results confirmed that the change in BMI from 1994–1995 to 2001–2002 was larger (0.9 kg/m² (95% CI 0.8 to 1.0) in men and 1.3 kg/m² (95% CI 1.2 to 1.4) in women) than from 2001–2002 to 2007–2008 (0.2 kg/m² (95% CI 0.1 to 0.3) in men and women). The most recently born had the largest increase (p<0.001).

Conclusions The mean BMI and the prevalence of obesity are still increasing in Tromsø, and the increase is strongest in the youngest age groups. However, the increase in BMI was less marked in the last period (from 2001–2002 to 2007–2008) than in the first period (1994–1995 to 2001–2002).