During the period of 1954-1986, 399 cases of haemorrhagic fever with renal syndrome (HFRS) were registered in Bulgaria with 63 (15.7%) deaths. Three hundred serum samples from 214 patients who had contracted the disease from 1957 to 1986 were investigated by indirect fluorescent antibody test (IF-AT). As antigen Vero-E6 cells infected with the Asian Hantaan virus were used, as well as lung sections from bank voles (Clethrionomys glareolus) infected with strains Udmurt and Kazan 6-Cg from the European part of the U.S.S.R. Specific antibodies were detected in 194 sera, i.e. in 90.6% of persons investigated: in 131 single serum samples and in 63 paired sera. The results of the serological studies which covered 53.6% of all known local cases showed the territorial distribution of the natural foci in this country and the aetiologic relationship with the HFRS virus from the European part of the U.S.S.R.