Introduction Ovine respiratory disease is frequently associated to high economic losses. It can be caused by several aetiological agents, difficulting the diagnosis and the implementation of effective therapeutic measures. The pathologist has a difficult, but very important role, in the diagnostic procedure, based on standard histomorphological classification of lung injury. Objectives The authors describe and submit for discussion a case of hyperacute pneumonia with high mortality, associated with abrupt climate changes. Medical History High mortality rate, affecting young lambs. At necropsy, dark and firm lungs, with hepatisation were observed, without other significant macroscopic lesions. Material and Methods Lung samples were fixed in 10 % buffered formalin solution and 3 μm thick sections were stained for routine histopathological diagnosis with Haematoxylin and Eosin. Results and discussion Macroscopic examination revealed acute congestion and haemorrhage, involving pleura. The severe pulmonary haemorrhage and congestion and the absence of cranioventral fibrinonecrotizing bronchopneumonia and fibrinous pleuritis, led to consider other causes namely pneumotoxicosis but no Pasteurella. Elongated cells with basophilic spindle-shaped nuclei, in alveoli were also seen. The oat cells, probably caused by leukotoxin from Mannheimia haemolytica, an opportunistic pathogen member of the normal bacterial flora of the nasopharynx and tonsils of ruminants, were crucial to the diagnosis of the present case of hyperacute pneumonia, despite the absence of fibrin. Conclusion The presence of characteristic oat cells contributed to the diagnosis of Mannheimia haemolytica, however microbiological tests are still needed to make a definitive diagnosis.