True idiopathic hypereosinophilic syndrome has been described in human beings and cats, but not in dogs. The syndrome is characterised by prolonged unexplained peripheral mature eosinophilia, the infiltration of many organs by eosinophils, organ dysfunction and a fatal outcome. This paper describes an idiopathic disseminated eosinophilic disease in a dog involving various organs, manly the heart and the lungs, accompanied by a leukemoid eosinophilic response, and a fatal outcome. The histopathological findings included the infiltration of the myocardium, lung parenchyma, liver, spleen, lymph nodes and skeletal muscles with eosiniphils.