Background and Objectives: Obesity has become a global pandemic, affecting millions of people worldwide. It is a known risk factors for several non-communicable diseases and caused mainly by unhealthy diet and sedentary lifestyle. The risk of developing obesity has also changed over time. However, less is known how it affects men and women in different birth cohorts. This study estimated how body mass index (BMI) distribution has changed among adult Finnish population over the past 20 years. Methods: Data from Finnish health examination surveys on adults was used. A series of cross-sectional surveys were conducted between 1997-2017 every 5-years among population living in mainland Finland covering age group 25-64 years. In all surveys, trained nurse measured height and weight using standardized protocol and calibrated measurement devices. The proportion of participants in different BMI categories was calculated by survey year, sex, age group, and birth cohort. Results: The prevalence of obesity has increased in both men and women over the 20 years being 24% for men and 21% for women in 2017. Change has not happened uniformly in the distribution of BMI. We observed that the proportion of normal weight population has remained relatively unchanged over the years but the proportion of population with obesity has increased. The upward shift of the upper tail of the BMI distribution has been the greatest among the age groups under 45 years, but among older people there has also been polarization development to the opposite direction in normal weight persons. Conclusions: Increase in obesity and especially polarization of BMI at young age is of concern and has significant impact on incidence of non-communicable diseases such as diabetes, cardiovascular diseases, and several cancers. This will cause increased demand for healthcare services and increase both direct health care costs but also indirect cost of informal care.