Abstract: Objective To study the effect of anti-virus therapy by monitoring the cytomegalovirus（CMV） level in peripheral blood of patients after hematopoietic stem cell transplantation（HSCT）. Methods CMV DNA in peripheral blood of patients after allogenic HSCT was regularly detected by quantitative real-time PCR.Of the 57 patients（38 males,19 females） at the age of 13-59 years（a median age of 31 years） who could be evaluated,30 had acute leukemia,11 had chronic myeloid leukemia（CML）,8 had non-Hodgkin lymphoma（NHL）,5 had myeloid dysplastic syndrome（MDS）,and 3 had severe aplastic anemia（SAA）.CMV DNA was detected once a week after HSCT.Anti-virus therapy started when the CMV DNA copy number was over 103 twice or over 104 once.The treatment was lasted till one week after CMV DNA became negative. Results Of the 57 patients,32（56.1%） were found to be positive for CMV DNA,the copy number of CMV DNA ranged 1.2×102-7.8×105.Of the 26 patients who received anti-virus therapy,11 died.Only one of the dead patients was found to be associated with CMV infection. Conclusion Regular detection of CMV DNA contributes to the timely treatment of viremia patients,decreases the occurrence of CMV disease,and prolongs their survival time.