Inhibitors represent one major complication of haemophilia treatment, as they increase the risk of bleeding, physical disability and mortality. The Cost Of Care Inhibitors Study (COCIS) showed that modern strategies applied to manage patients with inhibitors adsorb high amounts of resources but provide satisfactory levels of Health-Related Quality-of-Life (HR-QoL). This paper focuses on determinants of HR-QoL in inhibitory patients. Fifty adult patients, enrolled by 11 Italian Haemophilia Centres, were clinically assessed and filled in two HR-QoL generic questionnaires: the EuroQol instrument (EQ-5D) and the Short Form-36 (SF-36). According to our results, bleeding frequency and inhibitor titres were not found associated with HR-QoL. Global HR-QoL, and in particular the physical component of wellbeing in these patients was found negatively associated with their orthopaedic condition: the EQ-5D Visual Analogue Scale (P<0.001) scores, the SF-36 domain 'physical functioning' and 'physical component summary' (P<0.01) scores were found significantly correlated with the orthopaedic joint score, even after adjusting for patients' age. These results were confirmed by those from the EQ-5D profile. To conclude, the COCIS study is the first study showing that HR-QoL in inhibitory patients is impaired by their orthopedic status, while other aspects do not seem to influence patients' global wellbeing. Our results suggest that while the management of this complication is satisfactory, the attention has now to be focused on the prevention of the orthopaedic problems in these patients, which nowadays constitute one of the most important aspects to be considered in the haemophilia care.