One wild eastern cottontail (Sylvilagus floridanus) from Milwaukee County, Wisconsin was necropsied. The lungs contained numerous multifocal, circumscribed, tan foci; the spleen was markedly enlarged and had a mottled reddish tan color; and the brain had a red to tan friable tract in the left hemisphere. Microscopically, the lung had a severe bronchiolitis and pneumonia. The bronchiolitis was characterized by epithelial cells containing eosinophilic intranuclear inclusion bodies. The encephalomalacia of the left cerebral cortex featured tissue disruption and astrocytes or neurons containing intranuclear inclusion bodies. Herpesvirus particles were found within the bronchiolar epithelial cells. Based on histopathological and ultrastructural findings, a herpesvirus seemed the most likely etiologic agent.