Acute leukaemias develop quickly and need to be treated urgently. Chronic leukaemias develop more slowly and may not need to be treated for some time after they are diagnosed. Some forms may not require any treatment. Myeloid leukaemias arise from myeloid stem cells and are characterised by the accumulation of cancerous myeloid cells. Lymphoid leukaemias arise from lymphoid stem cells and are characterised by the accumulation of cancerous lymphoid cells such as B-cells and T-cells.
The most common forms of leukaemia in adults are CLL and AML, and the common cancer in children is ALL. Leukaemia is more common in adults.