Bone marrow transplantation (BMT) may change the recipient's pretransplant serostatus for herpes group viruses. We reviewed 68 patient records (1 year's transplants) to determine how frequently this occurs. Only 7 had data on serostatus before as well as at days 20 to 35 and greater than 100 after BMT. Serostatus was assessed by complement fixation and ELISA. All patients received a variety of blood-product support after BMT. One patient converted from anti-varicella zoster virus (VZV) positive to negative after BMT and developed clinical chickenpox; an additional patient converted from anti-VZV negative to positive after BMT but reverted to seronegative by day 102. One patient converted from anti-cytomegalovirus (CMV) positive pre-BMT to persistently anti-CMV negative post-BMT (donor: anti-CMV negative). Two patients had a greater than 4-fold fall in anti-herpes simplex (HSV) antibody post-BMT, and both shed HSV after BMT. Most seronegative recipients of marrow from seropositive donors developed herpesvirus antibodies. We conclude that the herpesvirus serostatus of BMT recipients should be determined again after BMT to aid in decisions about antiviral prophylaxis and diagnosis of clinical disease.