Neoplastic monocytoid B-cells (MBCs) are present in different amounts in several types of non-Hodgkin's lymphomas (NHLs), including monocytoid B-cell lymphoma (MBCL), splenic marginal zone lymphoma (SMZL), mucosa-associated lymphoid tissue (MALT) low-grade B-cell lymphomas, and follicular centroblastic-centrocytic (CB-CC) lymphomas. In an attempt to clarify the relationships between different groups of tumors with a significant monocytoid component, we studied six primary lymph node MBCL, three SMZL, seven MALT lymphomas, and four CB-CC with monocytoid differentiation. Their clinical, morphological, immunohistochemical and molecular features were compared. The results show wide overlapping between MALT and MBCL in terms of morphology, immunophenotype, and molecular features. Follicular colonization was a characteristic finding in both groups. Some MBCL revealed mucosal involvement during the course of the disease, suggesting a possible MALT origin. Our data support the suggestion that the use of the term MBCL should be discontinued in cases with mucosal involvement, as they are probably examples of lymph node involvement brought about by MALT lymphomas. Although SMZL have some overlapping features with MBCL and MALT lymphomas, some of the clinical and morphological specific findings justify their distinction from the other groups. The CB-CCs with monocytoid differentiation frequently harbored t(14;18), lacking any significant differentiating features from conventional follicular CB-CC lymphomas. Additional studies are needed to define the molecular features of MBCL and other marginal zone tumors.