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 (CMVAg-emia). 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 > or =0.5 x 109/l at a median interval of 16, 21, 17, 15 days after BMT, and Plt > or =30x10(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.