This study set out: (1) to describe resource utilization patterns among four groups of HIV-infected patients at different stages of the disease, and (2) to estimate the direct hospital costs of HIV/AIDS care among these patients in Italy. It is a multi-centre, prospective observational study conducted between August 1997 and July 1998. The 483 patients, enrolled in five infectious diseases departments located in different areas of Italy, were stratified into four groups according to their CD4+ lymphocyte cell count and status of AIDS Defining Illness (ADI) at enrolment. Average direct hospital costs (L = Italian Lire, 1997 exchange rate US$ 1 = 1,704 Italian Lire) were L.23,725,584 (US$ 13,923.5), L.15,208,287 (US$ 8,925.1), L.11,942,761 (US$ 7,008.7) and L.7,660,942 (US$ 4,495.9) for the four groups of patients. More than 80% of patients in the first group and about 65% of patients in the second group received highly active antiretroviral therapy (HAART). The proportion of patients receiving HAART in the third and fourth group increased from 37.1% to 56.5% and from 15.3% to 31.5%. The number of hospital days observed in this study was much lower than previously published numbers in Italy. These results may indicate a shift of costs from hospitalisation to outpatient care and ARV.