Background This study aimed to estimate annual health care and lost productivity costs associated with excess weight among the adult population in Belgium, using national health data. Methods Health care costs and costs of absenteeism were estimated using data from the Belgian national health interview survey (BHIS) 2013 linked with individual health insurance data (2013-2017). Average yearly health care costs and costs of absenteeism were assessed by body mass index (BMI) categories - i.e., underweight (BMI < 18.5 kg/m(2)), normal weight (18.5 <= BMI < 25 kg/m(2)), overweight (25 <= BMI < 30 kg/m(2)) and obesity (BMI >= 30 kg/m(2)). Health care costs were also analysed by type of cost (i.e. ambulatory, hospital, reimbursed medication). The cost attributable to excess weight and the contribution of various other chronic conditions to the incremental cost of excess weight were estimated using the method of recycled prediction (a.k.a. standardisation). Results According to BHIS 2013, 34.7% and 13.9% of the Belgian adult population were respectively affected by overweight or obesity. They were mostly concentrated in the age-group 35-65 years and had significantly more chronic conditions compared to the normal weight population. Average total healthcare expenses for people with overweight and obesity were significantly higher than those observed in the normal weight population. The adjusted incremental annual health care cost of excess weight in Belgium was estimated at euro3,329,206,657 (euro651 [95% CI: euro144-euro1,084] and euro1,015 [95% CI: euro343-euro1,697] per capita for individuals with overweight and obesity respectively). The comorbidities identified to be the main drivers for these incremental health care costs were hypertension, high cholesterol, serious gloom and depression. Mean annual incremental cost of absenteeism for overweight accounted for euro242 per capita but was not statistically significant, people with obesity showed a significantly higher cost (p < 0.001) compared to ...