The leukaemias and lymphomas are neoplastic diseases of the blood and blood-forming organs that are highly treatable and often curable. They cause bone marrow and immune system dysfunction, which renders the host highly susceptible to infection and bleeding. Often organ dysfunction secondary to tumour cell infiltration or tumour compression of vital structures may be apparent clinically. Tumour formation and organomegaly are more common with the lymphomas than with leukaemias.