In an open-labeled randomized study, prophylactic treatment with ganciclovir (day 15 to day 29) was administered to 23 cytomegalovirus seronegative patients who received a kidney from a cytomegalovirus seropositive donor. Both groups (control = 11, ganciclovir = 12) were similar in age, immunosuppressive treatments, acute rejection episodes and number of steroid pulses. A seroconversion occurred in 10 control patients (91 percent) and in 10 patients of the ganciclovir group (84 percent). A cytomegalovirus disease was observed in 10 control patients (91 percent) and in 8 patients of the ganciclovir group (66 percent). The delay between grafting and cytomegalovirus disease was significantly longer in the ganciclovir than in the control group (78.5 +/- 7.7 vs 46.5 +/- 5.5 days, P < 0.05). In conclusion, in renal transplant recipients who are at high risk of cytomegalovirus disease, prophylactic treatment with ganciclovir delays the onset of the disease and seems to decrease slightly its frequency.