A study of European brown hare syndrome (EBHS) was conducted in Poland (Czempin). From April 1993 until February 1994, 100 blood and 78 spleen samples of European brown hares (Lepus europaeus) were tested for prevalence of EBHS and rabbit hemorrhagic disease (RHDV) antibodies and EBHS virus antigen with two enzyme-linked immunosorbent assay (ELI-SA) test kits. Thirty-eight of 100 serum samples were positive for EBHS, and six (7.6%) of 78 of the spleen materials were-antigen positive for EBHS virus. Three (3%) of the sera were positive against RHDV, whereas two of these also were seropositive for EBHSV. European brown hare syndrome virus seropositive hares were most frequently found from April to September. Based on negative staining electron microscopy investigations of liver and spleen homogenates of all antigen-positive hares, we observed caliciviruses in only one animal. For histopathological investigations organ specimens were available from 98 hares. Histopathological findings corresponded with the clinical picture of chronic EBHS. A pathohistological picture consistent with EBHS was found in 22 (22%) of investigated hares and corresponded in 50% of the animals which reacted positively in the EBHSV antigen-ELISA and in 29% of the animals which reacted positively in the EBHSV antibody-ELISA. These results from western Poland are the first that caliciviruses are present in European brown hare population in Eastern Europe and may be one of the causes for increased mortality in the Polish hare population over the past 10 years.