Eleven patients underwent bone marrow transplant (BMT) from an HLA-identical sibling following single dose total body irradiation (TBI), with in vivo and ex vivo T cell depletion (TCD). In spite of CMV prophylaxis with acyclovir and high-dose i.v. Ig, 10 of 11 patients developed CMV antigenemia, at a median interval from BMT of 34 days (range 16-72 days) and five died with CMV disease. Foscarnet was then given prophylactically in 11 additional TCD patients to test whether we could (1) prevent CMV reactivation, and (2) reduce transplant-related mortality. Foscarnet was given daily from days +10 to +15 (180 mg/kg/day), then thrice weekly (90 mg/kg/day) until day +100. Five patients developed CMV antigenemia at a median interval from BMT of 42 days (range 16-65 days); one progressed to CMV pneumonitis and died. The risk of developing CMV antigenemia within day 100 is currently 91% for the historical control group and 45% for the foscarnet group (p = 0.005). At diagnosis of CMV, the median number of CMV antigen-positive cells was 6.5 (range 1-13) vs 1 (range 1-5) in acyclovir vs foscarnet patients (p = 0.02) and the median highest number of CMV antigen-positive cells was 7 (range 3-110) vs 1 (range 1-12), respectively, (p = 0.03). The actuarial 1 year transplant-related mortality (TRM) is 49% and 13% in the two groups (p = 0.08). This study suggests that foscarnet prophylaxis starting on day +10 post-BMT may be helpful in reducing the risk of CMV disease and early mortality following TCD BMT.