This is a report on the clinical signs of Wesselsbron disease in 37 lambs and the pathology of 4 natural and 12 experimental cases. Generally the symptoms were ill-defined. At autopsy 13 of the 14 lambs revealed a mild to severe icterus and a slight to moderate hepatomegaly with discoloration of the liver. No foci of hepatic necrosis were observed macroscopically and, except for petechial and ecchymotic haemorrhages in the mucosa of the abomasum and generalized lymphadenopathy, no other obvious macroscopic lesions were noted. Perforation of the abomasum occurred in one lamb only. Microscopy on the liver showed mild to extensive necrosis of the parenchyma. Degenerated and necrotic hepatocytes were diffusely scattered throughout the liver, but no definite well-circumscribed foci of necrosis were seen. Mitotic figures and hepatocytes with large nuclei indicated that active regeneration of parenchymal cells had occurred in some of the livers. Other changes, for example, Kupffer cell proliferation, sinusoidal leucostasis, bile duct proliferation and infiltration of mononuclear cells in the portal triads, were frequently encountered. Moderate to severe cholestasis was a feature in 66% of the livers examined, while intranuclear inclusions and intracytoplasmic acidophilic or Councilman-like bodies were frequently observed. A complete haematological study was carried out on 4 experimentally produced cases. The gross and histopathological lesions in the liver are compared with those of Rift Valley fever in the new-born lamb.