A survey of herpesvirus resistance to antiviral drugs was conducted by mailing a questionnaire to the centers of the European Group for Blood and Marrow Transplantation (EBMT). Results from 68 centers were reported. The number of centers with proven or suspected resistance of herpes simplex virus (HSV) was 17 (25%), of varicella-zoster virus (VZV) was three (4%), and of cytomegalovirus (CMV) was 19 (28%). Acyclovir-resistant HSV strains were recovered from 10 patients with HSV disease. Replacement of acyclovir by foscarnet in seven of these patients resulted in improved or healed HSV disease in five (17%). Acyclovir-resistant VZV was isolated from one patient with zoster which improved after a change to vidarabine therapy. CMV resistance to ganciclovir was proven in only two patients but was suspected in 23. Ganciclovir was replaced by foscarnet in 15 of these 25 patients which resulted in better virological and/or clinical outcome in 13 (87%). These results suggest that herpesvirus resistance is an emerging problem in marrow transplant recipients, and that foscarnet treatment may prove to be a valuable alternative when the presence of acyclovir- or ganciclovir-resistant herpesvirus disease is documented in these patients.