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Hairy cell leukemia (HCL) is a very rare and well-defined entity that is characterized by the presence of hairy cells expressing the characteristic markers CD11c, CD25, CD103 and CD123 and in most cases by the presence of the BRAF^V600E mutation. The incidence rate, directly adjusted to the 2000 United States standard population is estimated at 0.62 per 100 000 person-years (PA) in white men, 0.21 in black men, 0.20 in Asian men and 0.06 American Indian or Alaska native men. Based on the estimated 2019 leukemia incidence, of 61,780 in the United States, approximately 1,240 new HCL cases are expected per year, with only 60–75 new patients having a variant form of HCL each year. In France, the median age of patients at diagnosis is 63 years in men and 59 years in women. There is a strong male predominance, with a sex ratio of 5:1. HCL is a malignant disorder with a good prognosis, with a standardized average survival at 1 year and 5 years of 95% (95% CI: 91-97 for both). The etiology of HCL remains unknown. The risk of secondary cancers is high, especially that of another malignant hematologic disorder. This high risk justifies the need for prolonged hematological monitoring.

HCL must be distinguished from other HCL-like disorders, including the variant form of HCL (HCL-_V) or splenic diffuse red pulp lymphoma (SDRPL), warranting the development of international epidemiologic studies.