B-cell prolymphocytic leukemia is a very rare mature B-cell leukemia occurring in elderly people diagnosed when prolymphocytes comprise more than 55% of the lymphoid cells in peripheral blood. Its evolution is most often aggressive. We report here the cases of two patients with clinical signs dominated by splenomegaly associated with significant hyperlymphocytosis. Their blood smears revealed extensive prolymphocyte invasion suggesting Prolymphocyte leukemia confirmed by immunophenotyping. Both purine analogues and monoclonal antibodies have shown promoting results, however, allogeneic hematopoietic stem cell transplantation remains the only therapeutic means allowing a lasting response. However, it is reserved for young patients with compatible donors. A good understanding of the pathogenesis and a better integration of the molecular data of this hemopathy open the way to the use of new targeted therapies. Prolymphocyte leukemia is a very rare, complex, and often aggressive, mature lymphoid hemopathy. The mean age of onset is 65 years with a predominance of males. We report here the case of a patient with splenomegaly and severe lymphocytosis in whom the smear showed an invasion of 90% of prolymphocytes suggesting Prolymphocyte leukemia confirmed by immunophenotyping.