Leukaemias fall into 2 major groups: acute (fast growing) and chronic (slow growing). These are further classified according to the type of blood cell where the cancer begins:

• Acute lymphoblastic leukaemia (ALL), also called acute lymphocytic leukaemia, starts in the early form of lymphocytes, a type of white blood cell. This type of leukaemia develops rapidly. ALL is more common in children than in adults.
• Acute myeloid leukaemia (AML), also called acute myelogenous leukaemia, starts in myeloid cells (the cells that make red blood cells, platelets and white blood cells other than lymphocytes). It usually starts in the early form of white blood cells other than lymphocytes. This type of leukaemia develops rapidly. AML is most common in older people.
• Chronic lymphoblastic leukaemia (CLL), also called chronic lymphocytic leukaemia, starts in the early form of lymphocytes. Unlike ALL, the cancer cells build up slowly, and there may be no symptoms for several years. However, there is a second type of CLL that grows faster and is more serious. CLL mainly affects older adults. About two-thirds of leukaemias are CLL.
• Chronic myeloid leukaemia (CML), also known as chronic myelogenous leukaemia, starts in the early form of myeloid cells. It is slow growing, but can change into a fast-growing leukaemia. CML mainly occurs in adults. Only about 10% of leukaemias are CML.
• Hairy cell leukaemia starts in the lymphocytes. The name of the disease comes from the hairy appearance of the leukaemia cells under a microscope. This type of leukaemia gets worse slowly or not at all. It may not require treatment for many years.
• Chronic myelomonocytic leukaemia (CMML) results in a large number of monocytes (a type of white blood cell) in the blood. About 15–30% of people with CMML go on to develop AML.