We have investigated the cellular origin and/or pathogenesis of follicular small cleaved cell lymphoma (FSCCL), diffuse small cleaved cell lymphoma (DSCCL) and intermediate lymphocytic lymphoma/lymphocytic lymphoma of intermediate differentiation (ILL/IDL) based on a series of immunologic and molecular genetic (bcl-1, bcl-2 and bcl-3 genes) studies. These studies have led to the conclusion that the cellular origin or pathogenesis of ILL/IDL and DSCCL is distinctly different from that of FSCCL: (1) FSCCL is a neoplastic counterpart of follicular center cells (FCC) of secondary follicles because of the presence of CD10 and bcl-2 gene rearrangement and the absence of CD5 and bcl-1 gene rearrangement; (2) DSCCL and ILL/IDL are a neoplastic counterpart of mantle zone (MZ) B lymphocytes because of the presence of CD5 and bcl-1 gene rearrangement and absence of CD10 and bcl-2 gene rearrangement; and (3) FSCCL scarcely develops into DSCCL, and the previously proposed concept that DSCCL represents a diffuse counterpart of FSCCL does not hold good. These results indicate that DSCCL and ILL/IDL are identical, derived from primary follicular cells or MZB cells of secondary follicles, and should be unified under MZB lymphocyte-derived lymphomas. They are distinguished from FCC-derived lymphomas in morphologic, immunologic, cytogenetic and molecular genetic features. Bcl-1 and bcl-2 genes may be associated with the pathogenesis of FCC-derived lymphoma and MZB lymphocyte-derived lymphoma, respectively.