Patients are randomized in a 2:1 ratio to receive either oral ganciclovir or placebo for a minimum of 12 months. PER AMENDMENT 9/19/94: Patients who have not reached a study endpoint may choose to continue blinded prophylaxis or discontinue blinded prophylaxis and begin open-label ganciclovir. PER AMENDMENT 5/2/95: After the common closing date (6/3/95) patients who have not met a CMV end point or experienced a serious toxicity that required permanent discontinuation of active oral ganciclovir will be eligible to receive open-label oral ganciclovir through an open-label extension phase of study 023 until 8/31/95.