Acute leukaemias are caused by the body making large numbers of immature blood cells. They cause symptoms such as anaemia, tiredness, infections and bruising or bleeding. They are fast growing and usually need to be treated as soon as possible. Treatment is often with intensive combinations of chemotherapy drugs.

Chronic leukaemias develop slowly. They are caused by the build up of abnormal mature white blood cells in the body. The symptoms of tiredness, infections and anaemia are similar to those of acute leukaemia but often it takes months or even years for these to develop. In some cases chronic leukaemia may not need to be treated for months or years after it is diagnosed. And, when treatment is given, it may be with a low dose of just one chemotherapy drug. So, treatment is usually less intensive than for acute leukaemia.

Blood cells start off as called stem cells. These special cells can develop into any type of blood cell. But, they need to go through lots of stages of development before they are mature cells.

Blast cells can divide rapidly to make copies of themselves. But, normally they only do this when our bodies need more of a particular type of cell. Mature cells can't divide to make new cells. They do their job and when they are old, damaged or no longer needed by us they die and are cleared from our bodies.

In acute leukaemia the cell that has become abnormal is an immature blast cell. So it is able to make lots of copies of itself very quickly. Because of this symptoms develop rapidly. In chronic leukaemia the abnormal cell is at a much later stage of development. So it isn't able to make copies of itself quickly. Instead the problem is caused by the abnormal cells not dying when they should. So, gradually over time the number of abnormal cells gradually grows until they begin to cause symptoms.

People of any age, including children, can develop acute leukaemia. But, chronic leukaemias are commoner in older people.