We investigated changes in the prevalence of obesity among Finnish adults (aged ≥30 years) during a 20-year period. Data were derived from two cross-sectional nationally representative surveys (n = 13 844) in 1978–1980 and 2000–2001. Weight and height were measured using a standardized protocol. Obesity was defined as body mass index (BMI) ≥ 30 kg m−2. Cut-offs of BMI ≥ 35 kg m−2 and BMI ≥ 40 kg m−2 were also used. A 20-year difference in the prevalence of obesity was from 11.3% to 20.7% in men and from 17.9% to 24.1% in women. In 1978–1980, 1.1% of men and 3.8% of women had a BMI at least 35 kg m−2. The corresponding prevalence was 3.9% in men and 6.8% in women 20 years later. The educational gradient in obesity diminished in 20 years because of the most prominent increase among highly educated men. Yet, 25% of men and 28% of women with low education are obese. Obesity increased in all age and educational groups over the 20-year period. It was highest among women and individuals with the lowest education, but the increase was most striking among well-educated men. A comprehensive public health strategy targeting the whole population and especially those with low education is urgently needed to halt the obesity epidemic.