We describe a patient with T-cell prolymphocytic leukemia who developed a large-cell lymphoma of the T-cell type. The neoplastic cells of the large-cell lymphoma demonstrated the same immunologic phenotype (ER+, Leu-3+, Leu-2-, and HLA ABC+) as the cells of the prolymphocytic leukemia, which indicated a clonal evolution of a T-cell prolymphocytic leukemia into a large-cell lymphoma of the T-cell phenotype.