Chronic lymphocytic leukaemia (CLL) is a cancer of the lymphocytes. It is the most common type of leukaemia. CLL mainly affects people over 60 and is rare in people under the age of 40.

CLL occurs more often in men than women, but the reason for this is not known.

Blood cells are normally produced in a controlled way, but in leukaemia the process gets out of control. The lymphocytes multiply too quickly and live too long, so there are too many of them circulating in the blood. These leukaemic lymphocytes look normal, but they are not fully developed (immature) and do not work properly. Over a period of time the abnormal cells replace the normal white cells, red cells and platelets in the bone marrow.

The condition usually develops very slowly and many people with chronic lymphocytic leukaemia do not need treatment for months or years. Some people need to have treatment straight away.