By application of combined structural and functional analyses, most lymphoid neoplasms may be categorized as of T- or B-cell origin. T lymphocyte neoplasms include types of acute and chronic lymphocytic leukemias, certain cutaneous and node-based lymphomas, and lymphomas of thymocytes (convoluted lymphocytic lymphomas). Although much less frequent than B-cell neoplasms, these T-cell neoplasms are important because their recognition has therapeutic and prognostic significance. Relatively specific histopathologic, histochemical, and immunologic criteria have been defined for each neoplasm. These neoplasms are also significant because homogeneous populations of T neoplastic cells have been used successfully in a few cases to study the normal biology of the immune system.