Leukaemias are:

• malignant neoplasms of the haemopoietic stem cells
• primary neoplasms of the bone marrow

The malignant cells gradually replace the normal bone marrow and may spill over into the peripheral circulation.

Leukaemias may be divided into:

• acute leukaemias:
• show rapid proliferation of relatively undifferentiated malignant cells
• are characterised by the development of bone marrow failure with consequent anaemia, neutropenia, and thrombocytopenia
• are the most common leukaemias among children

• chronic leukaemias:
• have a relatively prolonged natural history
• are characterised by better differentiated malignant cells
• compromise marrow function at a late stage
• have features of the large load of malignant cells e.g. hepatosplenomegaly
• are rare in children

Leukaemia is the most common childhood malignant disease - with an incidence of 3.5 per 100,000 per year in children under the age of 15 years. But as a whole, it is more common in the elderly - 60% occuring in patients over 50 years of age.