Chen YP, Yen YS, Chen TY, Yen CL, Shieh CC, Chang KC. Systemic Mycobacterium kansasii infection mimicking peripheral T-cell lymphoma. APMIS 2008;116:850–8. Nontuberculous mycobacteria are opportunistic pathogens which predominantly infect the immunocompromised host. The clinical and pathologic diagnosis of mycobacterial infection is generally not difficult. However, it may mimic malignancy on account of the clinical manifestations or the morphology of atypical lymphocytes with epithelioid histiocytes. The latter can be found in some types of lymphomas, especially T-cell lymphoma. This report describes two immunocompetent patients with systemic Mycobacterium kansasii infection presenting with fever, systemic lymphadenopathy, and osteolytic bone lesions. The microscopic features of these two cases were similar and were characterized by effacement of the nodal architecture by lymphocytic infiltrates and small aggregates of epithelioid histiocytes throughout. These lymphocytes showed mild atypia and expressed predominantly CD3. Bone marrow was also involved in the same process in one case and T-cell lymphoma with lymphoepithelioid features was the initial impression. However, further studies reported germline arrangements of T-cell receptor genes, presence of acid-fast bacilli, and recovery of M. kansasii in culture. At follow-up, the lymphadenopathy was seen to have disappeared during antimycobacterial treatment. This report describes two infectious cases with small aggregates of epithelioid histiocytes and atypical lymphocytes mimicking peripheral T-cell lymphoma; and such cases may become more common as the number of immunosuppressed hosts is increasing worldwide. We have reviewed the literature and summarized useful morphologic criteria for differentiation.