In this report two dogs with essential thrombocythaemia (ET) are described. Both dogs were presented more or less at the same time with a combination of reduced exercise tolerance and pale mucous membranes without any report of blood loss. Moderate-to-severe, Coomb's-negative anaemia and thrombocytosis (> 1249 x 10'/l) were present. In addition, the peripheral blood smear revealed the presence of basophilia and large numbers of abnormally shaped megakaryocytes in the bone marrow of both dogs. Treatment with vincristine (0.7 mg/m2 once intravenously) and hydroxyurea (500 mg/m2 p.o. per day) was started. Because of insufficient response to treatment after 3 weeks, the dosage of hydroxyurea was increased in both dogs to 2000 mg/m2 p.o. per day. The dogs deteriorated further, however, and were euthanized at 6 weeks after the start of treatment. Blood examination revealed pancytopenia in both dogs, most likely due to the myelosuppressive effects of high-dose hydroxyurea. A survey of veterinary literature on ET is presented, including a comparison of ET in humans.