Objective This study aimed to analyze the clinical manifestations of human herpesvirus 6(HHV-6)infection within 100 days after allogeneic hematopoietic stem cell transplantation(allo-HSCT)and to investigate the association of HHV-6 viral load with clinical outcomes as well as the effect of antiviral treatment on the course of HHV-6 infection.Methods This retrospective study included patients who tested positive for HHV-6 within 100 days after allo-HSCT at the Peking University Institute of Hematology from February 2016 to February 2023.The study analyzed the patients'baseline characteristics,including age and transplantation type,as well as their clinical manifestations.Additionally,post-transplant complications were examined.Results This study detected that 305 patients with HHV-6 infection were positive with a median time of 20 days post-transplant.Fifteen patients were asymptomatic,whereas the remaining patients exhibited the following symptoms:fever,rash,diarrhea,hemorrhagic cystitis,delayed platelet engraftment,central nervous system symptoms,abdominal pain,pneumonia and perioral numbness.Acute graft-versus-host disease(aGVHD)was diagnosed in 189 patients,with 45 cases of HHV-6 infection occurring before the onset of aGVHD and 120 cases occurring after aGVHD developed.Quantitative HHV-6 detection was available for 45 patients,and no statistically significant differences were found in the clinical manifestations according to the viral titer.A total of 108(35.41%)patients experienced coactivation with other viruses,including cytomegalovirus,BK virus,and Epstein-Barr virus(EBV).Notably,coinfection with EBV was determined as an independent risk factor for overall survival(OS).No statistically significant difference in the time to HHV-6 viral clearance was observed between the antiviral treatment and non-treatment groups[7(5-10)days vs 8(4-14)days,P=0.199].Similarly,the 5-year OS rates between the two groups were not significantly different[(82.7±2.6)%vs(91.3±3.1)%,x2=3.304,P=0.069].Discussion The most prevalent clinical manifestations were fever,rash,and diarrhea in patients with HHV-6 infection after allo-HSCT.No significant correlation was found between the severity of the clinical symptoms and the viral titer.Additionally,no significant differences in the time to HHV-6 clearance or 5-year OS were observed between patients who received antiviral treatment and those who did not.