This paper estimates the impact of BMI, obesity and overweight on direct medical costs. We apply panel data econometrics and use a two-part model with a longitudinal dataset of medical and administrative records of patients in primary and secondary healthcare centres in Spain followed up over seven consecutive years (2004- 2010). Our findings show a positive and statistically significant impact of BMI, obesity and overweight on annual medical costs after accounting for data restrictions, different subsamples of individuals and various econometric approaches.