A seven-year-old flat-coated retriever presented with a history of lethargy, dyspnoea and inappetence of several days' duration. Clinical examination revealed pale mucous membranes and tachypnoea, and haematology demonstrated marked autoagglutination. Thoracic radiographs revealed an increased opacity in the perihilar region. The owners declined further evaluation and the dog was treated symptomatically with immunosuppressive doses of prednisolone and azathioprine. The dog's demeanour improved, although it was eventually euthanased seven weeks later because of dysphagia and worsening dyspnoea. Postmortem examination revealed a widespread, poorly differentiated sarcoma involving the lungs, pericardium, thoracic lymph nodes and spleen. Immune-mediated haemolytic anaemia is a well recognised condition in dogs and is occasionally associated with neoplastic conditions. This is the first case report to describe immune-mediated haemolytic anaemia associated with a diffuse, poorly differentiated sarcoma.