Chronic lymphocytic leukaemia is a slowly progressing (chronic) malignant blood disease where morphologically normal looking B lymphocytes accumulate in the bone marrow, blood and lymphoid tissue (lymph nodes, spleen), leading to leucocytosis, lymphocytosis and, in some cases, to enlarged lymph nodes and/or splenomegaly. The clonal lymphocyte population gradually displaces the normal healthy haematopoiesis in the bone marrow. The subsequent bone marrow failure will lead to anaemia, neutropenia and/or thrombocytopenia. The diseased cells exhibit characteristic chromosomal changes, which have formed as a result of acquired mutations. The disease is not hereditary. In chronic monoclonal B-cell lymphocytosis (MBL), blood lymphocytes frequently carry surface antigens typical to CLL, i.e. they have the immunophenotype of CLL, but the number of lymphocytes is only slightly increased (less than 5 x 109/l) and other cell counts are normal. MBL is not considered a malignant condition, but the patient should, however, be monitored (for example, annually) because in some cases MBL may progress to CLL.