Objectives: Although there is a comprehensive public health insurance system in Belgium, out-of-pocket expenditures can be very high, mainly for inpatients. While a large part of the official price is reimbursed, patients are confronted with increased extra billing (supplements). Therefore, the government imposed various restrictions on the amount of supplements to be charged, related to the type of room and the patient’s insurance status. We investigate how prices are set and whether the restrictions have been effective. Methods:We use an administrative dataset of the Belgian sickness funds for the year 2003 with billing data per hospitalisation and hospital characteristics. Boxplots describe the distribution of several categories of supplements. OLS is used to explore the relationship between hospital characteristics and extra billing. Results: There is a large and intransparent variation in extra billing practices among different hospitals. Given the room type, supplements per day are smaller for patients qualifying for protection, confirming that the regulation is applied quite well. However, because of their longer length of stay this does not result in lower supplements per stay for these patients. Conclusions: Currently the price setting behavior of providers lacks transparency. Protective regulation could be refined by taking into account the longer length of stay of vulnerable groups.