Oral ganciclovir is effective in preventing cytomegalovirus (CMV) disease in HIV-infected patients despite a bioavailability of only 6-9%. To determine safety, pharmacokinetics, and the influence of acute gastrointestinal graft-vs.-host disease (GI-GVHD) on the bioavailability and antiviral effect of oral ganciclovir after marrow transplantation, CMV seropositive patients received oral ganciclovir (1000 mg 3 times per day) from day 35 (+/- 7 days) until day 100 after transplantation. Single-dose (intravenous and oral) and steady-state oral pharmacokinetic profiles and weekly trough levels were performed. Twenty-one patients received oral ganciclovir (seven with GI-GVHD, 14 without); 17 had steady-state pharmacokinetic profiles and seven had single-dose profiles. The absolute bioavailability was similar in patients with or without acute GI-GVHD (7.2 vs. 6.9%). At steady state, the extent and rate of absorption of oral ganciclovir were comparable in these same patient subgroups (area under the curve [AUC] = 13.5 and 10.2 mg x hours/L, respectively; time to peak serum ganciclovir concentrations = 5.5 and 3.8 hours, respectively). Breakthrough CMV antigenemia, viremia, or plasma polymerase chain reaction positivity occurred in eight of 21 (38%) patients (four of seven with GVHD and four of 14 without). Drug discontinuation because of GI adverse effects was required in six of 21 (29%) patients. Neutropenia occurred in two of 15 (13%) patients who had received oral ganciclovir for more than 10 days. In conclusion, the bioavailability of oral ganciclovir seems similar to that reported in other settings. The presence of acute GVHD of the GI tract did not appear to adversely affect absorption of oral ganciclovir. The use of oral ganciclovir was limited by the presence of GI intolerance in the early posttransplant period. The efficacy of oral ganciclovir in preventing CMV infection in marrow transplant recipients is being assessed in a separate randomized controlled trial.