Denileukin diftitox (Ontak(®)) is indicated for the treatment of patients with persistent or recurrent cutaneous T-cell lymphoma (CTCL), a rare lymphoproliferative disorder of the skin. Denileukin diftitox was the first fusion protein toxin approved for the treatment of a human disease. This fusion protein toxin combines the IL2 protein with diphtheria toxin, and targets the CD25 subunit of the IL2 receptor, resulting in the unique delivery of a cytocidal agent to CD-25 bearing T-cells. Historically, immunotherapy targeting malignant T-cells including monoclonal antibodies has been largely ineffective as cytocidal agents compared to immunotherapy directed against B-cells such as rituximab. This review will summarize the development of denileukin diftitox, its proposed mechanism of action, the pivotal clinical trials that led to its FDA approval, the improvements in quality of life, and the common toxicities experienced during the treatment of patients with CTCL. CTCL is often a chronic progressive lymphoma requiring the sequential use of treatments such as retinoids, traditional chemotherapy, or biological response modifiers. The incorporation of the immunotoxin denileukin diftitox into the sequential or combinatorial treatment of CTCL will also be addressed.