Twenty-five patients with hematologic malignancies (n = 21) or aplastic anemia (n = 4) undergoing an allogeneic BMT from an HLA-identical sibling developed cytomegalovirus (CMV) antigenemia at a mean interval from BMT of 41 days (range 16-141 days). All patients were treated at the time of antigenemia in the absence of other signs of CMV disease with ganciclovir (n = 13) or foscarnet (n = 12) if the WBC count was < 2.5 x 10(9)/l or the patient had aplastic anemia. The two groups were comparable for age, sex and disease status. There were more patients receiving T cell-depleted grafts in the foscarnet group (58% vs 15%, p = 0.003). The first course of treatment was planned to last a minimum of 10 days: foscarnet was given at 180 mg/kg/day, and ganciclovir at 10 mg/kg/day. Patients still showing pp65-positive cells continued treatment in the absence of adverse effects such as cytopenia and/or increased creatinine levels. Maintenance treatment was given for 3-4 weeks. End-points of the study were (1) clearing of CMV antigenemia, (2) tolerance and side-effects, and (3) progression to CMV disease. Both agents were effective in clearing CMV antigenemia: 14 of 25 patients were CMV antigen-negative by day 14 of treatment and all surviving patients were negative by day +50. Renal toxicity was seen mainly in the foscarnet group but caused discontinuation of the drug only in one patient. Myelotoxicity was seen in the ganciclovir group and again could be controlled in 12 of 13 patients.(ABSTRACT TRUNCATED AT 250 WORDS)