Since the discovery of human T-cell leukemia virus type 1 (HTLV-1) in patients with adult T-cell leukemia/lymphoma (ATLL), malignant neoplasms of mature (peripheral) T lymphocytes have attracted a great deal of attention. This type of neoplasm is more common in Japan than in Western countries, and may show distinct clinical pictures such as hypergammaglobulinemia, hypercalcemia, etc. T-cell lymphomas are more prone than B-cell lymphomas to become leukemic. Because of a marked intermingling of reactive cells (histiocytes, eosinophils, etc.), the histologic diagnosis of T-cell lymphoma is often difficult. Proliferation pattern and cellular size do not correlate with prognosis as in B-cell lymphoma. Since T-cell lymphomas often manifest with several distinct clinicopathologic settings, their categorization should be based on several parameters, such as the presence or absence of ATLL-associated antigen in serum, histology, phenotype of the neoplastic cell, and clinical features. Since a classification for T-cell lymphomas has not been established, a further multi-disciplinary approach is necessary for a better understanding of this interesting neoplasm.