Cyclin-dependent kinase-6 (CDK6) is the earliest inducible member of the CDK family in human T lymphocytes, involved in growth factor stimulation and cell cycle progression. CDK6 is one of the targets of p16 and p15, CDK inhibitors encoded by MTS1 and MTS2, two tumor suppressor genes that are frequently deleted in T-cell leukemia. In this study we have investigated CDK6 expression in normal and neoplastic lymphoid tissues using immunohistochemistry and flow cytometry. In normal (six samples) and hyperplastic (four samples) thymuses, strong CDK6 expression was observed in a discrete proportion of cortical thymocytes (10 to 15%), mainly located in the peripheral (subcapsular) zone of the cortex. All tested cases of T-cell lymphoblastic lymphoma/leukemia (T-LBL/ALL) showed strong CDK6 expression in the majority (up to 100%) of neoplastic lymphoid cells. Western blot analysis confirmed the expected CDK6 protein size (40 kd). According to Southern blot analysis, CDK6 overexpression in neoplastic T lymphoblasts was not due to gene amplification. In all other lymphomas investigated (28 peripheral T-cell non-Hodgkin's lympohomas (T-NHLs), 7 CD30+ anaplastic NHLs, 22 high-grade B-NHLs, 15 low-grade B-NHLs, 25 B-cell precursor ALLs), CDK6 was not expressed or expressed at low levels, with the only exception of three nasal angiocentric T-NHLs, all exhibiting CDK6 immunoreactivity comparable to that observed in T-LBL/ALL. These data provide evidence that CDK6 is abnormally expressed in T-LBL/ALL and may be involved in the pathogenesis of this malignancy. In addition, the quantitative difference of CDK6 expression between neoplastic and non-neoplastic cortical thymocytes can be potentially useful in the differential diagnosis of thymic neoplasms on histological and cytological specimens.