Human herpesvirus 7 (HHV-7) is widespread around the world and may also be a possible cofactor for cytomegalovirus (CMV) infection in haematopoietic stem cell transplant (HSCT) recipients. In case of viral diseases where specific treatment is available, real-time PCR assays constitute reliable diagnostic tools enabling timely initiation of appropriate therapy and rapid assessment of the efficacy of antiviral treatment strategies. The presence of CMV and HHV-7 was confirmed by the detection of viral DNA isolated from 1,027 plasma samples. A group of 69 allogeneic HSCT (alloHSCT) recipients was examined in early post-transplant period using quantitative real-time PCR methods. Within the study period, 62 % of patients had at least once CMV DNA-emia, while HHV-7 DNA was found in 43 % of subjects. Co-infection between these β-herpesviruses was detected in the plasma samples collected from 18 patients (26 %). Patients with concomitant HHV-7 DNA-emia had significantly higher number of CMV DNA copies compared with those without HHV-7 infection (1986 vs. 432 copies/ml, p < 0.001) but there was no difference in duration of CMV DNA-emia between these groups. On the other hand, while the load of HHV-7 DNA was comparable between patients with CMV DNA-emia and without CMV DNA-emia, the duration of HHV-7 DNA-emia was significantly longer in the first group (38.5 vs. 14 days, p < 0.001). HHV-7 DNA-emia is very frequently detected in Polish alloHSCT recipients. In those, who have subsequent CMV reactivation, the coexistence of the viruses may negatively affect the kinetics of infection with either of them. Therefore the investigation of concomitant HHV-7 DNA-emia could affect the prognosis of post-transplant patients suffering from CMV reactivation.