This paper estimates the increase of direct medical costs of both severe and moderate obesity and overweight with respect to a normal-weight individual using a two-part generalised linear model and a longitudinal dataset of medical and administrative records of patients in primary and secondary healthcare centres followed up over seven consecutive years (2004-2010) in Spain. Our findings indicate that severe and moderate obesity imposes a substantial burden on the Spanish healthcare system. Specifically, being severely obese is associated with increases in medical costs of 26% (IV estimate: 34%) compared to a normal-weight individual. The effects of moderate obesity and overweight are more modest raising medical costs by 16% (IV estimate: 29%) and 8.5% (IV estimate: 23%), respectively. These changes in costs are slightly higher for those patients below the median age and for the women. Notwithstanding, the effects found in this study are comparatively much lower than that reported for the US, based basically on a private healthcare system and characterised by a more obese population.