Aim: The present study aimed to describe histopathological findings and immunohistochemical expression of proliferating cell nuclear antigen (PCNA) in a spontaneous case of pulmonary adenomatosis in an adult ram. Case Presentation: A carcass of an adult ram was presented for necropsy in the Post-mortem Hall of the Department of Veterinary Pathology, College of Veterinary Sciences of the Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar with a history of respiratory distress. A detailed necropsy was performed. The tracheal mucosa showed mild congestion. The lungs were pale and firm in consistency. On histopathological examination, nasal turbinate and trachea both showed the vascular changes along with mild infiltration of mononuclear cells. The sections of lungs revealed pulmonary adenocarcinoma characterized by the presence of cuboidal to low columnar epithelial cells forming acinar or papillary-like structures in the parenchyma and surrounded by the normal alveoli. Areas of necrosis along with infiltration of mononuclear cells were also evident. An Immunohistochemical study of the lung sections revealed mild to moderate immunopositive reactivity for the PCNA in the nuclei of neoplastic cells of adenocarcinoma. It indicated the proliferative activity of neoplastic cells. Conclusion: Based on histopathological examination, the present case was diagnosed as pulmonary adenomatosis. PCNA expression indicated the presence of actively proliferating neoplastic cells.