This chapter discusses erythropoietin (EPO) excess, a condition characterized by an overproduction of red blood cells (RBCs). The chapter highlights the importance of considering the differences in EPO structure between humans and animals when using immunoassays to measure EPO concentrations. It further describes the different types of erythrocytosis, including relative and absolute, and primary and secondary. Guidelines are provided for diagnosing and treating erythrocytosis in dogs and cats, and the prognosis and potential complications associated with the condition are discussed.