&NA;B‐cell prolymphocytic leukemia (B‐PLL) is a rare lymphoproliferative disorder with clinical, morphologic, and immunophenotypic features that overlap with other mature B‐cell leukemias and lymphomas. Patients with B‐PLL typically have marked splenomegaly and leukocytosis, have no significant lymphadenopathy, and have a clinically aggressive course and poor prognosis. In peripheral blood (PB) and bone marrow (BM) aspirate smears, numerous large prolymphocytes with a round to oval nucleus and a single large nucleolus are observed. The French‐American‐British group recommends requiring that >55% prolymphocytes be present in the PB (or BM) to establish the diagnosis of B‐PLL. In BM biopsy sections, B‐PLL extensively replaces the medullary space in a pattern that is diffuse, interstitial, or both. Immunophenotypic studies have shown that B‐PLL patients express monotypic immunoglobulin light chain (of bright intensity) and pan‐B‐cell antigens, and variably express the CD5 antigen. Cytogenetic studies have reported several chromosomal abnormalities, which are usually complex. The differential diagnosis of B‐PLL is reviewed.