OBJECTIVE: Although the immigrant population in cities such as Barcelona has tripled in the last five years, until now the impact of this group on the health system has not been rigorously evaluated. The aim of this study was to compare hospital resource utilization among the immigrant population with that among the native population through case mix, demographic characteristics and hospital day use. MATERIAL AMD METHODS: We analyzed 15, 057 discharges from Hospital del Mar in Barcelona in 2000. This hospital attends 60% of admissions from the Ciutat Vella district. In 2000, 21% of the population of this district were immigrants. Socio-demographic patient characteristics and case mix were compared between the immigrant and the native population. Hospital resource use was compared according to age, case mix (diagnosis related groups) and seriousness (severity, complications and comorbidities) of the events requiring medical care. RESULTS: The case mix of the immigrant population differed from that of the autochthonous population due to pronounced ge differences and a higher fertility rate. Thirty-three percent of immigrant admissions were for deliveries. The mean cost of discharge of immigrants from low-income countries was 30% lower than that for the remaining discharges. After adjusting for age, case mix and severity, length of stay among the immigrant population was significantly shorter. A 5% reduction was found after adjusting for case mix and a 10% reduction was found when all the factors were considered. CONCLUSIONS: Case mix differences are due to age and socio-cultural factors. Immigrants are rejuvenating the ageing native population and the role of gynecology-obstetrics and pediatrics needs to be increased. The finding that resource use per discharge is lower among immigrants from low-income countries contradicts the expectation that lower socioeconomic status leads to higher hospital resource use intensity. Therefore, new hypotheses and analyses that explain this situation should be put forward.