Infectious mononucleosis (IM) is an acute lymphoproliferative disorder that typically occurs in young patients and is usually caused by Epstein-Barr virus. We report here, two cases of tonsillar lesion of IM resembling marginal zone B-cell lymphoma mucosa-associated lymphoid tissue (MALT) type. The patients consisted of an 18-year-old Japanese woman and a 36-year-old Japanese man. Both patients presented with tonsillar mass. Histologically, in one case, the tonsil showed diffuse proliferation of medium-sized lymphocytes. The medium-sized lymphocytes had round or slightly indented nuclei with a small solitary nucleoli and abundant clear cytoplasm and somewhat resembled monocytoid B-cells. In the remaining one case, the lymphoid follicles had hyperplastic germinal centers with ill-defined borders surrounded by a sheet-like proliferation of polymorphous infiltration showing a marginal zone distribution pattern. On high-power field, the interfollicular area was diffusely infiltrated by a polymorphous infiltrate of medium-sized lymphocytes with angulated nuclei somewhat resembling centrocyte-like cells, immunoblasts, plasma cells, plasmacytoid cells and histiocytes with or without epithelioid cell feature. However, there were no CD43+ B-cells in either lesion. Moreover, the polytypic nature of the B-cells was demonstrated by immunohistochemistry or polymerase chain reaction. Although MALT type lymphoma rarely affected young adults, notably, a number of cases have been reported in the tonsil. The present two cases indicated that acute IM should be added to the differential diagnosis for MALT type lymphoma in young adults.