Mature or peripheral T-cell lymphomas are uncommon, accounting for only 10%-15% of all non-Hodgkin's lymphomas. The classification of these neoplasms has been controversial. In contrast to B-cell lymphomas, cytologic grade has not been very useful in predicting the clinical course. This finding may result from the generally aggressive clinical course associated with T-cell lymphomas. Prior studies have suggested that stage of disease may be more important than cytologic subtype. Clinical presentation is very important in the classification of T-cell malignancies. For T-cell lymphomas, cytologic features alone are not sufficient to distinguish among disease entities. For example, adult T-cell leukemia/lymphoma (ATLL) often cannot be distinguished morphologically from HTLV-1-negative T-cell lymphomas. Most extranodal T-cell lymphomas appear to be derived from cytotoxic T cells, which express perforin, TIA-1, and granzyme B. Three broad groups of T-cell malignancies can be identified: (1) leukemic or systemic disease; (2) nodal disease; (3) extranodal disease. Anaplastic large-cell lymphoma (ALCL) is probably the single most common subtype of T-cell lymphoma. Classical ALCL should be distinguished from primary cutaneous ALCL (CD30+ lymphoproliferative disease of the skin), which is a distinct disease entity.