The incidence of chronic lymphocytic leukaemia varies with the age and sex structure of the population. Analysis of the Surveillance, Epidemiology, and End Results (SEER) database notes the US incidence as being 3·5 per 100 000 per year (men 5·0, women 2·5). In the Leukaemia Research Fund data collection study, researchers gathered data from individual haematologists responsible for laboratories covering about a third of the population of England and Wales and reported an incidence of chronic lymphocytic leukaemia in the UK of 6·15 per 100 000 per year, although this value concealed a variation between 1·3 and 13·7 per 100 000 per year in different health districts, dependent largely on how interested the local haematologist was in the disease. Since, in our experience, more than three-quarters of patients with chronic lymphocytic leukaemia are discovered because of an incidental blood count, the exact prevalence of the disease depends clearly on how assiduous is the case finding.
Chronic lymphocytic leukaemia is rare in people younger than 50 years, but after this age a fairly rapid rise in incidence takes place. According to SEER data, the median age for diagnosis of the disease is 70 years for men and 74 years for women, and median age at death is 76 years and 81 years, respectively. White American individuals have a slightly higher incidence than those of African-American origin (3·9 vs 2·8 per 100 000 per year), but in American people of Chinese, Japanese, and Filipino extraction, incidence is about five times lower, even in those who have adopted a fully American lifestyle. Early data put the incidence of chronic lymphocytic leukaemia in Jewish people at twice that of non-Jewish North American individuals.
Chronic lymphocytic leukaemia can arise in families. and  First-degree relatives of patients with the disease are three times more likely to have chronic lymphocytic leukaemia or another lymphoid neoplasm than the general population. With a four-colour flow-cytometric assay, Rawstron and colleagues  noted that 3·5% of healthy individuals older than 40 years had a population of monoclonal lymphocytes in their blood, with the immunophenotypic characteristics of chronic lymphocytic leukaemia cells, at concentrations lower than 3·5×109/L; in first-degree relatives of patients with familial chronic lymphocytic leukaemia, the prevalence of such cells is between 13·5% and 18%. and  The relation between subclinical chronic lymphocytic leukaemia and full-blown disease is a matter of intense investigation in several laboratories.
No consistent evidence is available to link chronic lymphocytic leukaemia with environmental exposure to either radiation or chemicals, except in the case of agricultural workers and herbicides. On Jan 23, 2003, the US National Academy of Sciences' Institute of Medicine published a report concluding that there is “sufficient evidence of an association between exposure to Agent Orange, a herbicide used in Viet Nam, and the development of chronic lymphocytic leukaemia”.  Although ionising radiation has traditionally been absolved from causing chronic lymphocytic leukaemia, recent studies have suggested that this may be unwarranted.
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11 Committee to review the health effects in Vietnam veterans of exposure to herbicides (fourth biennial update), Veterans and Agent Orange: 2002, National Academic Press, Washington, DC (2003), pp. 373–377.
12 TJ Hamblin, Have we been wrong about ionizing radiation and chronic lymphocytic leukemia?, Leuk Res 32 (2008), pp. 523–525.