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Mature B-cell lymphoproliferation with hairy lymphocytes include splenic marginal zone lymphoma (SMZL), hairy cell leukemia (HCL), splenic diffuse red pulp lymphoma (SDRPL), and hairy cell leukemia variant (HCL-v), the latter two being provisional entities that appeared in the 2008 WHO classification. We report the case of a 75-year-old man who benefited from a diagnostic re-evaluation of his SMZL. The good clinical evolution, the flow cytometry investigation (HCL score 3, strong CD180 and CD200/CD180 ratio < 0.5), and the histological assessment favored a diagnosis of SDRPL. This entity did not exist at the time of the man’s diagnosis in 2006. The differential diagnosis between these diseases is sometimes not straightforward. Here we detail some practical avenues to take to help reach an accurate diagnosis.