Small lymphocytic lymphoma (SLL), mantle cell lymphoma (MCL), and SLL plasmacytoid (SLLP) are malignant neoplasms of small B cells that may have overlapping cytologic features. Entities such as SLL with irregular nuclear contours may pose additional diagnostic difficulties. We investigated the utility of flow cytometric analysis and immunohistochemistry studies in distinguishing these disorders from each other. We reviewed 29 lymphomas and classified them as SLL (13 cases), MCL (8 cases), and SLLP (8 cases) based on histology and expression of cytoplasmic immunoglobulin light chain. Paraffin section immunohistochemistry was performed for CD5, CD20, CD23, CD43, CD45RA, CD45RO, and kappa and lambda light chains. Flow cytometric analysis was carried out by 2-color direct immunofluorescence for CD5, CD11c, CD19, CD20, CD22, CD23, FMC7, and kappa and lambda light chains. By immunohistochemistry, we found that the expression of CD23 in SLL discriminates between SLL and MCL and that the expression of CD23 and CD43 in SLL discriminates between SLL and SLLP. By flow cytometric analysis, we found that CD11c+ and dim fluorescence intensity of slg and dim fluorescence intensity of FMC7 in SLL distinguish SLL from MCL, and the expression of CD5+, CD23+ and dim fluorescence intensity of FMC7 in SLL distinguishes SLL from SLLP. We found no immunophenotypic difference between SLL and SLL with irregular nuclear contours.