Since October 1986 an unusually high mortality has been observed both in wild European brown hares (Lepus europaeus) and in hare farms in Italy. Pathological alterations, including severe hepatosis, nephrosis, congestion and hemorrhages of tracheal mucosa and lungs, and splenic vascular congestion, were observed in 179 of 381 hares necropsied from 1986 to 1988. Jaundice also was seen in 30% of these hares. Histologically, the liver damage was characterized by coagulative necrosis, mainly located around the portal areas, or by degenerative changes. Hyperemia, focal hemorrhages and periportal mononuclear cell infiltration were also present. The epithelium of renal tubules showed the presence of various degrees of vacuolar degeneration and necrosis, and eosinophilic granular hyaline casts or homogenous proteinaceous material were found within the lumen of tubules. Only the adult hares were affected. In wild hare populations night counts revealed a reduction of the number of observed wild hares during the winter period which ranged from 27 to 40%, whereas in hare farms the mortality ranged from 30 to 90%. Bacteriological, parasitological, and toxicological investigations were unable to confirm the primary cause of these deaths. Negative stain electron microscopy and immunoelectronmicroscopy conducted since October 1988 on liver and spleen homogenates from hares with acute hepatosis revealed the presence of viral particles with morphological aspects resembling those of calicivirus, antigenically related to the etiological agent of viral haemorrhagic disease of rabbits.