Hospitals’ incentives to provide health care are influenced by the degree of cost sharing between the purchaser and the provider. In most OECD countries, governments remunerate hospitals according to the activity performed. Activity is usually measured through a diagnosis related groups (DRGs) system. This study estimates the degree of cost sharing of the NordDRG classification system (the DRG version of the Nordic countries) in Iceland during 2003–2005. We first apply ordinary least square (OLS) methods to estimate the degree of cost sharing by regressing the price for each individual patient against its cost. Second, we propose an instrumental-variable approach to address the potential endogeneity of costs. The OLS estimates suggest that the degree of cost sharing is in the range 0.16–0.17 (i.e. 1increaseincostisassociatedwithanincreaseinreimbursementby0.16–0.17 ). The instrumental-variable approach provides some evidence of endogeneity, and suggests that cost sharing is overestimated by OLS. The instrumented estimates of cost sharing are 0.11, 0.13 and 0.14 in 2003, 2004 and 2005, respectively. Regardless of the method applied, most cost sharing is associated with the retrospective features of the classification system (such as the type of treatment provided).