Sirolimus (rapamycin), in combination with standard immunosuppressive therapy, significantly reduced the incidence of acute rejection (AR) in a prospective trial in kidney transplant recipients. The purpose of this study was to determine the relationship between resource utilization and the number of AR episodes. METHODS: Medical resource utilization data were collected for 719 US patients enrolled in a randomized, double blind, controlled clinical trial. Patients were categorized into three groups according to their experience of AR (both biopsy confirmed and presumptive) during the first 6 months post transplant: those who had no AR (N=542), one AR episode (N=125), and two or more AR episodes (N=52). Measures of medical resource utilization were compared between groups. The measures included mean hospital days, days on dialysis, and amount of concomitant immunosuppressive rescue medications. Different immunosuppressive rescue drugs were combined into a single measure in the analysis using cost estimates based on average wholesale prices. RESULTS: Patients experiencing one AR episode had higher resource utilization rates (18.8 hospital days, 10.6 days on dialysis, and $10,600 cost of concomitant immunosuppressive rescue drugs) compared with those experiencing none (12.6 hospital days, 1.7 days on dialysis, and $1,400 of rescue drugs). Patients with multiple AR episodes had the highest rates of resource utilization (20.5 days of hospitalization, 10.4 days on dialysis, and $16,600 of concomitant immunosuppressive rescue drugs). All three-way differences were significant at the .05 level based on F-tests. CONCLUSIONS: AR episodes in kidney transplant patients were associated with higher utilization of resources during the first six months post-transplantation. A reduction in the AR rate by sirolimus may offer both improved economic as well as clinical outcomes.