Summary:This is a dose-finding study using foscarnet for CMV prophylaxis after allogeneic bone marrow transplantation (BMT) in 20 high risk patients (unrelated donors, or T cell depleted, and/or advanced disease). Foscarnet was started on day ؉1 after BMT and continued until day ؉100. We explored four different dose levels, patients being entered at the lowest dose level until one patient experiences CMV-reactivation, identified as two consecutive positive CMV antigenemias (CMVAgemia). The four dose levels expressed as mg/kg/day between days 1 and 30 (induction) and between days 31 and 100 (maintenance) were respectively: dose level I ‫؍‬ 60/30 (n ‫؍‬ 5); dose level II ‫؍‬ 120/60 (n ‫؍‬ 4); dose level III ‫؍‬ 120/90 (n ‫؍‬ 5) and dose level IV ‫؍‬ 120/120 (n ‫؍‬ 6). All patients showed engraftment: PMN Ն0.5 ؋ 10 9 /l at a median interval of 16, 21, 17, 15 days after BMT, and Plt у30 ؋ 10 9 /l on days 19, 16, 17, 17 respectively. CMVAg-emia was seen in 10 patients at a median interval of 53 days post-BMT (range 33-89) with a median of 10 CMV antigen؉ cells (range 1-16). There was a dose effect of foscarnet on CMVAg-emia: respectively 4/5 patients (80%), 2/4 (50%), 3/5 (60%) and 1/6 (18%) at dose levels I, II, III, IV (P ‫؍‬ 0.1). CMV disease was seen in 3/9 (33%) at dose levels I, II and 0/11 at dose levels III, IV (P ‫؍‬ 0.07). The median number of CMV antigen-positive cells at diagnosis of CMV infection was different: 13 in dose levels I-II and two in dose levels III-IV (P ‫؍‬ 0.01). Increased creatininine was seen in 15 patients with a mean of 1.8 mg% (range 1.5-5.7) and was the cause of discontinuation in nine patients (45%). Renal toxicity was reversible in all nine patients. Overall actuarial TRM at 2 years was 31%: 47% for patients at dose levels I-II and 19% for patients at dose levels III-IV. In conclusion, foscarnet exhibits a dose-dependent prophylactic effect on CMVAg-emia, CMV disease and transplant-related mortality with acceptable and reversible renal toxicity. Bone Marrow Transplantation