The purpose of the study To estimate the prevalence of markers of herpesvirus infections and pneumocystosis in patients of the hematological department. To identify whether the contamination of hematological patients with hepatitis B and C (HBV, HCV) affects on the activation of opportunistic infections (OIs). Materials and methods There were examined 94 patients of the hematological department, including 48 patients with markers of HBV infection and 46 cases without them. Obtained results Markers of acute OIs (simple herpes virus type 1- 6,4%, EBV - 14,9%, CMV - 6,4%, and Human Herpesvirus 6- 13,8% and 33,0% of pneumocystosis) in hematological patients were determined more frequently than in donors. Comparative analysis of the data revealed the excess of the proportion of cases with markers of acute HHV-6 infection in hematological HBV patients in comparison with a group of hematologic patients without markers of HBV infection (22,9% against 4,4%, p<0,05; χ2=6,8). Conclusion In the performed study there was shown the statistically significant effect of HBV and HCV in the activation of infection caused by HHV-6, whereas in relation to other herpes viruses (HSV, CMV, EBV), and pneumocystosis, this regularity has not been proved. At the same time in patients with hematological malignancies the proportion of the contamination with opportunistic infections was established to be higher than in donors. Opportunistic infections markers were detected almost at the same level in both groups of patients: patients with HBV, HCV markers, and cases without markers.