The system for real-time polymerase chain reaction was developed for detection of Puumala virus RNAand quantitative analysis of viral RNAload in clinical samples. Dynamics of viral load in blood samples from 12 patients with HFRS (from day 4 to day 17 of the disease) was studied. At day 4 viral RNA load varied from 4 x 10(4) to 1 x 10(6) copies/ml. During the period of observation viral load decreased by 10 - 200 times and was not detected in majority of patients to day 11 - 15 of the disease. The hantavirus persisted in blood of four patients to day 17 of the disease with viral load varied from 2.7 x 10(3) to 2 x 10(4) copies/ml. Developed method was characterized by highest sensitivity (100%) for HFRS detection from day 4 to day 8 of the disease.