In veterinary oncology the use of the nitrosourea compound lomustine is increasing. Veterinary oncologistsneed to be aware of the pulmonary toxicity of this drug.Because of the lack of veterinary publications on this subject, the incidence and pathophysiology in humancancer patients of pulmonary toxicity induced by cytotoxic agents in general and by nitrosoureas in particularare discussed. Three clinical syndromes can be recognized, the most devastating of which is interstitialpneumonitis resulting in pulmonary fibrosis. Disturbances in the homeostatic mechanisms of the oxidant/antioxidant-,immunologic-, matrix repair-, proteolytic-, and central nervous systems are some of the major mechanismsof pulmonary injury in human medicine. Risk factors such as cumulative dose, age, radiation, oxygenadministration and multi-drug regimens are recognized.For the first time in veterinary medicine, a case report of a dog with pulmonary fibrosis, probably causedby chronic lomustine administration, is presented.