The present study compared foscarnet with ganciclovir for preemptive therapy of cytomegalovirus (CMV) infection after allogeneic blood or marrow stem cell transplantation (SCT). Patients with CMV infection, as detected by weekly antigenemia or polymerase chain reaction (PCR) in blood leukocytes, were randomized to intravenous therapy for 2 weeks with either foscarnet at 60 mg/kg or ganciclovir at 5 mg/kg administered every 12 hours; if CMV infection remained detectable, patients received an additional 2 weeks of intravenous foscarnet at 90 mg/kg or ganciclovir at 6 mg/kg given once daily for 5 days per week, after which therapy was stopped. Primary efficacy endpoint was the occurrence of CMV disease or death from any cause within 180 days after SCT. A total of 213 patients were treated with either foscarnet (n = 110) or ganciclovir (n = 103). Kaplan-Meier estimates of event-free survival within 180 days after SCT were similar in the 2 treatment groups (P = .6). During study treatment, severe neutropenia (< 0.5 × 10⁹/L) occurred in 11 (11%) patients on ganciclovir versus 4 (4%) patients on foscarnet (P = .04), and impaired renal function was observed in 5 (5%) patients on foscarnet versus 2 (2%) patients on ganciclovir (P = .4). Neutropenia or thrombocytopenia required discontinuation of ganciclovir in 6 (6%) patients but in no foscarnet-treated patient (P = .03). After allogeneic SCT, preemptive therapy of CMV infection with foscarnet shows similar efficacy as with ganciclovir, but is associated with a lower proportion of patients who develop severe neutropenia and who require discontinuation of antiviral therapy due to hematotoxicity.