A 10.5-year-old, castrated male shih tzu was presented for evaluation of weakness, pica, and pallor of the mucous membranes. A hemogram indicated an inflammatory leukogram and a regenerative anemia with spherocytosis and thrombocytosis. The dog responded well to conservative therapy for immune-mediated hemolytic anemia (IMHA). However, the thrombocytosis did not resolve. Serial hemograms were characterized by persistent thrombocytosis (platelet count, 577,000 to 1,200,000/microl) with abnormal platelet morphology. A systematic investigation ruled out causes of physiological and reactive thrombocytoses. A diagnosis of essential thrombocythemia was made by fulfilling the criteria of the Polycythemia Vera Study Group of the National Cancer Institute. The marked thrombocytosis was nonresponsive to hydroxyurea therapy. The dog remains healthy despite the marked increase in the number of circulating platelets. A review of causes of thrombocytoses in humans and animals is presented, and the criteria for diagnosis of essential thrombocythemia are examined.