ABSTRACTThe clinical findings, laboratory data, ultrastructure, and autopsy findings are reported for four Greyhounds with a myeloproliferative disorder. In all dogs abnormalities were found in the granulocytic, erythroid and megakaryocytic cell lines, resulting in granulocytic leukaemia, impaired erythropoiesis and megakaryocytic dysplasia. The presence in blood of many small platelet producing cells (micromegakaryocytes) led to initial diagnostic confusion in three dogs with two of them suspected of having lymphatic leukaemia. An approach to the diagnosis of myeloproliferative disease, including the awareness of other clinical possibilities, is presented. The significance of the presence of micromegakaryocytes in the peripheral blood stream is discussed with reference to other veterinary reports and similar findings in man.