OBJECTIVES: The ERANET-PRIOMEDCHILD RaDiCEA Project is an international, multicentre, longitudinal observational study evaluating the effectiveness and the economic dimension of the ultra-rare cryopyrin associated periodic syndromes (CAPS) in patients with the indication for either use of anti IL-1 agent or other than anti IL-1; the present study presents results of interim analysis about the cost-of-illness (COI) of 11 patients (6 females, age range: 1-43 years) with CAPS from two Italian centres. METHODS: COI analysis was performed considering a bottom-up approach. Resource consumed estimations was based on clinical and economic data collected retrospectively and prospectively by ad-hoc questionnaires administered to patients or their proxy (on direct health and non-health costs, indirect costs) and to the budget and management control officer at each centre to collect information about the healthcare system type, hospitalization reimbursement system, payment for drugs outside the hospital setting, payment of special schools and devices, exemption, disability pension and/or allowance for caregivers. All costs were referred to Euro 2013 and presented as median [25-75 percentile] cost per patient/year. RESULTS: Overall costs per patients/year were 107,993 [60,392-302,305] Euro. Direct costs related to drugs consumption, hospitalization, laboratory and instrumental examination were about 106,911 [25,726-284,878] Euro, being mainly attributable to the cost of drugs (99%). Non-health direct costs for transportation and informal assistance were 124 [9-144] Euro and 735 [319-1.105] Euro respectively. Among adults patients only one was currently working and another one was unable to work; 6 patients needed informal assistance and caregivers lost about 28 working hours at week. CONCLUSIONS: Interim results showed that CAPS imply a significantly economic burden for the society with costs associated to drug being the main determinant. Final analysis will clarify the relative weight of the different cost component and test the methods in different centres/countries.