Obesity continues to increase in the majority of the population in mid-Sweden-a 12-year follow-up. 
The aim was to investigate trends in the prevalence of obesity by age and level of education in the general population in mid-Sweden from year 2000 to 2012.  A postal questionnaire was sent to a random population sample aged 25-74 years in years 2000, 2004, 2008 and 2012.  The overall response rates were 67%, 65%, 60% and 53%, respectively, and the study included 29 017, 27 385, 25 910 and 24 152 respondents, respectively.  Obesity (BMI ≥ 30 kg/m(2)) was based on self-reported weight and height.  The age-standardized prevalence of obesity increased from 13% to 17% in women and from 12% to 17% in men between 2000 and 2012.  Obesity increased in all age groups from 2000 to 2008 and continued to increase among the middle aged (45-64 years) between 2008 and 2012.  The socioeconomic gradient in obesity changed during the study period since the absolute increase in obesity was steepest at the middle educational level.  In 2012, the prevalence of obesity was almost twice as high at both middle and low educational levels compared with high educational level.  The 'true' prevalence of adult obesity, corrected for self-reported weight and height, was around 20% in 2012 for both men and women.  In the majority, among the middle-aged and those with secondary education, the prevalence of obesity continued to increase even between 2008 and 2012.