Our study objective was to determine the cost-effectiveness of prophylaxis with cytomegalovirus immune globulin for preventing CMV-associated disease in renal transplant patients. We used a decision analytic model applied to 5 groups of renal transplant recipients at varying risks of developing CMV-associated disease. We obtained the rates of developing CMV-associated disease, graft rejection, and mortality, and the effectiveness of CMV-IG from the published literature. We used actual variable costs of CMV-IG, hospitalization, dialysis, and outpatient follow-up. The incremental cost of administering CMV-IG compared with withholding it ranged from $29,800 per life saved for the highest risk group, CMV-seronegative recipients of kidneys from CMV-seropositive cadaveric donors, to $1.68 million per additional life saved for the lowest risk group, CMV-seronegative recipients of grafts from CMV-seronegative donors. The outcome was somewhat sensitive to the effectiveness of CMV-IG in preventing CMV-associated disease but not sensitive to wide variations in CMV-IG costs, dialysis costs, outpatient costs, and the probability of graft rejection. Our conclusion is that prophylaxis with CMV-IG is very worthwhile for renal transplant recipients at high risk of CMV-associated disease and is possibly worthwhile for some patients at lower risk. The cost-effectiveness of other strategies for preventing CMV-associated disease, such as prophylaxis with acyclovir, CMV vaccine, or unselected immune globulin, should be compared with CMV-IG.