Six cases of non-Hodgkin B-cell lymphoma that mimicked either chronic lymphocytic leukemia (CLL) or a CLL variant at presentation are reported. The patients ranged from 54 to 89 years and included three females and three males. All six patients had prominent peripheral blood lymphocytosis at presentation; the initial morphologic impression was CLL in three cases, CLL/prolymphocytic leukemia (PLL) in two cases, and PLL in one. Five patients had bone marrow biopsies; each showed a lymphoid infiltrate in a focally random, interstitial, and/or diffuse pattern. Flow cytometric immunophenotyping showed CD20-positive B cells with surface immunoglobulin (Ig) light chain restriction in all six patients. The five cases resembling CLL or CLL/PLL had at least a subset of CD5-positive B cells, whereas CD5 was absent in the one case that resembled PLL. CD23 was positive in three of the four cases studied that resembled CLL or CLL/PLL; CD79b was positive in three, FMC7 was positive in two, and surface Ig and CD20 were brightly positive in three. A t(11;14) (q13;q32) was found in four cases that resembled CLL or CLL/PLL; they were subsequently diagnosed as mantle cell lymphoma. The remaining two cases mimicking CLL or PLL were diagnosed as lymphomas of follicle center origin with leukemic phase based on the presence of t(14;18) (q32;q21). Thus although the morphology of these six cases resembled CLL or variants, and immunophenotyping by flow cytometry showed overlapping features, genetic studies enabled distinction of these leukemic non-Hodgkin lymphoma from chronic lymphocytic leukemia or variants.