Leukemias are a type of myeloproliferative disorder that is considered a rare occurrence in horses. They are characterized by the presence of neoplastic haemopoietic cells in the circulation, and can occur in acute or chronic form.

Acute leukemia occurs when there is neoplastic transformation of stem cells or early precursors and a subsequent release of poorly differentiated neoplastic cells into the circulation. Chronic leukemia occurs with neoplastic transformation of late precursors and release of mature differentiated cells. Leukemias are categorized as lymphoid or myeloid depending on their cellular origin.