Gleevec in Relapsed/Refractory T Cell Non-Hodgkin's Lymphoma

Inclusion Criteria: * Histologically confirmed T NHL, excluding T prolymphocytic leukemia, T lymphoblastic lymphoma, and T/NK large granular lymphocytic leukemia. * Measurable disease, defined as at least one bidimensionally measurable site of disease measuring at least 1.5cm in greatest diameter. * Failed at least one systemic chemotherapy or biologic therapy for T cell lymphoma unless it can be clearly documented that the patient can not tolerate such therapy. * 18 years of age or older * Life expectancy of greater than 3 months * ECOG Performance Status of lesser then or equal to 2 * Normal organ and marrow function as outlined in the protocol * Agree to the use of adequate contraception prior to study entry and for the duration of the study Exclusion Criteria: * Chemotherapy or radiotherapy within 4 weeks prior to entering the study * Receiving any other study agents * CNS lymphoma requiring active therapy * History of allergic reactions attributed to compounds of similar chemical or biologic composition to imatinib mesylate * Participants requiring concomitant administration of any medications or substances that are inhibitors or inducers of CYP3A4 are ineligible * Patient previously received radiotherapy to 25% or greater of the bone marrow * Uncontrolled intercurrent illness including but not limited to ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements * Pregnant or lactating women * History of a different malignancy except for individuals who have been disease-free for at least 5 years and are deemed by the investigator to be at low risk for recurrence of that malignancy * HIV-positive individuals on combination antiretroviral therapy * Known chronic liver disease * Major surgery within 2 weeks prior to study entry