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