the 5-year cancer prevalent rate in South Korea (1,522.7 per 100,000 population) was more than double the international prevalence (625 per 100,000 population; GLOBOCAN, 2012b). The number of cancer survivors and the related burdens are increasing with population ageing, advances in medical treatments and cancer screening programmes.High body mass index (BMI) is widely recognised as one of the most important modifiable risk factors for cancer. According to the Organization for Economic Co-operation and Development (OECD), Korea is a country where obesity rates are expected to increase at a rapid rate (Organisation for Economic Co-operation and Development, 2017). In particular, the prevalence of obesity in middle-aged people over 40 years of age is higher than the prevalence of obesity in young adults (Ministry of Health and Welfare, 2015). Obesity is becoming a social problem that leads to premature death, and it is causing healthcare costs to rise (Korea Health Abstract Objective: Cancer is a major cause of the burden of disease, and obesity is widely recognised one of the most important modifiable risk factor of cancer. Considering the economic impact of obesity and cancer, it is necessary to measure the economic burden of cancer attributable to excess body mass index (BMI). Methods: This study used medical check-up sample cohort data of National Health Insurance Service (NHIS) claims and during 2002-2015. To estimate the costs (direct and indirect) according to obesity-related cancer sites, we performed a Cox proportional hazard model and cost of illness (COI) methods.Results: Among male obesity-related cancer sites, the largest total costs caused by overweight or obesity were 5.5 trillion USD for liver cancer, 1.8 trillion USD for colorectal cancer and 1.6 trillion USD for kidney cancer. Among women, post-menopausal breast, liver and colorectal cancers had the largest total costs attributable to excess BMI (breast: 3.7 trillion USD, liver: 2.3 trillion USD, colorectal: 2.1 trillion USD). Conclusions:Approximately, 4.5% and 15.8% of total costs in obesity-related cancers can be reduced in men and women respectively. This study's findings highlight the importance of improved interventions, which can yield healthier lives and economic benefits beyond simply reducing cancer incidence and mortality. K E Y W O R D Scancer burden, cost-of-illness, economic burden, obesity