Ganciclovir, an acyclic nucleoside with improved activity against cytomegalovirus in vitro, was used to treat 15 marrow transplant patients with symptomatic cytomegalovirus infection of the gastrointestinal tract. Eleven of the 15 had improvement in one or more of their clinical signs or symptoms during treatment. No clinical relapses were observed. Viral excretion from throat, urine and blood stopped at a median of 6 days of treatment, but six patients had recurrence of viral excretion 7-25 days after treatment was stopped. The only toxicity was the development of reversible neutropenia in eight of 15 patients after 10-19 days of treatment. Neutropenia was not related to duration of treatment, plasma drug levels or to the neutrophil count at the beginning of treatment. Although treatment with ganciclovir may be associated with marrow suppression, the serious nature of gastrointestinal infection due to cytomegalovirus in the immunocompromised host, the antiviral effect and the possible clinical improvement observed in vivo, and the lack of other effective treatments justify further controlled studies with this agent in immunocompromised patients with serious cytomegalovirus infection.