The financial budget for public health care in Italy has been more and more restricted in the last few years, but, on the other hand, the care of AIDS patients is still very expensive and antibiotic therapy plays an important role in the management and cost of these patients. The antibiotic therapies and related costs have been evaluated in 99 patients affected by AIDS (59 pts), ARC (28 pts) or HIV serum positive (12 pts), all hospitalized or treated in Day Hospital for different bacterial infections in 1995 at the Department of Infectious Diseases, Ospedale Maggiore, Bologna, for a total of 7733 days of antibiotic therapy. The average cost for antibiotic therapy was about 400,000 Italian Lira, with no significant difference depending on the stage of HIV related disease. The crude cost for antibiotic treatment was not particularly high, but the high frequency of adverse events, registered in these patients, required additional medical support and/or a prolonged hospital stay, which increased substantially the total cost of management of bacterial infections.