To establish the incidence of CMV viremia after allogeneic blood stem cell transplantation, we studied 51 consecutive allogeneic peripheral blood stem cell (PBSC) transplant recipients. A total of 12 recipients were at moderate risk for CMV disease and 39 were at high risk. Conditioning regimens varied, but GvHD prophylaxis consisted of tacrolimus and mini-methotrexate in all patients. All patients received prophylactic ganciclovir from admission until day -2 and prophylactic acyclovir from day -1 until day 180 after transplantation. CMV viremia was treated with ganciclovir. Using a PCR-based technique, the cumulative incidence of CMV viremia was 31+/-14% by day 100 and 35+/-14% by day 150. Donor type, CMV risk group, underlying disorder, conditioning regimen, GvHD, and steroid use were not associated with the risk for CMV viremia. No cases of CMV disease occurred. We hypothesize that the low rate of CMV viremia and the absence of CMV disease in this cohort of PBSCT transplant recipients, which contrasts with other reports, may be related to the prophylactic use of high-dose acyclovir and possibly to pretransplant use of ganciclovir.