UNLABELLED Bosnia and Herzegovina has been known as a highly endemic region for Hantavirus infections for more than 50 years. Previous studies have shown that at least two different hantaviruses, the murine Dobrava (DOB) and avricoline Puumala (PUU) viruses, each carried by a different rodent species, have been circulating in the area. However, there is little information on rodent population density fluctuations in Bosnia over the past years as well as on the ratio of Puumala to Dobrava infection in humans. THE AIMS THE AIMS OF OUR STUDY WERE to identify the rodent species which may serve as hantavirus reservoirs in the north-east and central Bosnia; to assess the geographical distribution, density and population dynamics of rodent species in the area; to assess the influence of climatic conditions on the size of rodent population; and to determine the ratio of Puumala to Dobrava infection in humans. METHODS The epidemiologic and epizootic study in the north-east and central Bosnia was conducted during the 8-year period (1995-2003). The average yearly and monthly temperatures, air humidity and precipitation during the study period were analyzed. A total of 381 small rodents were caught during the epidemic years (1995 and 2002), and in-between the epidemic periods (1999 and 2000). The animals were caught by live-traps and identified by morphometric methods. The density of animals was estimated by counting the number of holes per 1000 m2. Sera of 311 patients with clinical signs and symptoms of hemorrhagic fever with renal syndrome (HFRS) were tested for the presence of antibodies reactive to the Dobrava, Puumala and Seoul viruses by using indirect immunofluorescence test (IIF), and IgG and IgM ELIS. Sera of 84 patients were tested using only IIF, and 227 sera were tested by IIF and -capture IgM ELIS tests. RESULTS During the epidemic years, the average monthly temperatures in February were by 4.3 times higher than the average temperatures during the nonepidemic years, which may have influenced the early reproduction of rodents and development of "mouse years". The rodents were identified as: Apodemus flavicollis (n = 139), Apodemus sylvaticus (n = 89), Apodemus agrarius (n = 4), Clethrionomys glareolus (n = 117), Sorex araneus (n = 5), Pytimus subterraneus (n = 23), Mus musculus (n = 1), Mycrotus arvalis (n = 1) and Rattus norvegicus (n = 2). Clethrionomys glareolus was predominant in the regions with the altitude higher than 1160 meters and Apodemus species in the regions with the altitude lower than 670 meters. The rodent population density changes seasonally and cyclically. During the epidemic years, the rodent population density was marked as very high, whereas during the nonepidemic years it was designated from low to moderate. Well-known natural hosts of Hantaviruses (A. flavicolis and C. glareolus) are most widely spread species of small rodents, and the increase in their population is closely related with outbreaks of epidemics of HVBS-a. Puumala virus caused HVBS-a in 49.84% (155/311); Dobrava virus in 23.15% (72/311) of cases, whereas Hantaviruses serotype was not identified in 27.00% (84/311) of cases. Infections caused by Puumala virus were more frequent than the infections caused by Dobrava virus during both epidemic and nonepidemic periods. The proportion of humans infected with Puumala and Dobrava viruses correlated with the number of natural hosts of Hantaviruses in the areas of HVBS outbreaks. The study of the prevalence of hantavirus antibodies in the populations of rodents and humans, which had been under way, should elucidate these relationships.