In an attempt to decrease the prevalence and severity of cytomegalovirus (CMV) disease, preemptive therapy with ganciclovir was administered to all renal transplant patients treated with OKT3 between February 1993 and December 1994 (26 patients). The results were compared with those of a historical group treated with OKT3 but not with ganciclovir (29 patients). Both groups were similar in age, sex, number of previous transplants, number of rejections, serological status of donor and recipient and OKT3 dose. Ganciclovir was administered during the period of treatment with OKT3. Only 2 (7.7%) treated patients developed CMV disease versus 11 (37.9%) of the control group (p = 0.01). In the control group the intensity of the disease was severe in 7 (63.6%) cases, whereas in the treated patients it was always of slight intensity (p = 0.01). In conclusion, preemptive therapy with ganciclovir during treatment with OKT3 decreases the prevalence and severity of CMV disease.