Background and Design: Mycosis fungoides (ME) is a cutaneous T cell lymphoma. The clinical and histopathological diagnosis of early mycosis fungoides is usually difficult. There is no special laboratory method for the diagnosis of ME and this is the most important problem in diagnosis and follow up the effectiveness of treatment. Adenosine deaminase (ADA) activity is a non-specific marker of T cell activation. In the present study, we aimed to investigate the levels of plasma and tissue ADA in patients with mycosis fungoides and to determine if ADA is an activation criterion for this disease.