We investigated the epidemiology of the European brown hare syndrome (EBHS) in Finland among free-living mountain hares (Lepus timidus) and European brown hares (Lepus europaeus). Carcasses of 297 European brown hares and 843 mountain hares submitted for postmortem examination were analyzed between 1990 and 2002. One hundred thirty-six cases of EBHS were detected during the examination, 20 cases in European brown hares and 116 in mountain hares. Liver samples from 56 histopathologically positive and 135 negative cases were analyzed by reverse transcription polymerase chain reaction (RT-PCR). The results of RT-PCR correlated well with histopathology. Cases of EBHS appeared only sporadically in a restricted area in eastern Finland during 1990–97. In 1998, the number of cases increased, and the disease spread to the north. In 1998–99, EBHS was the most common disease of mountain hares in necropsy material from northern and eastern Finland. Special features were the occurrence of the disease in areas in which European brown hares are rare and the seasonal distribution of cases, most of which appeared in spring or summer. This is the first report of an outbreak of EBHS occurring mainly among free-living mountain hares from Finland.