Alefacept, a LFA-3/IgG1 fusion protein, interferes with the activation and proliferation of T cells by binding to the CD2 receptor on their surfaces. The clinical efficacy of this drug has been demonstrated in chronic plaque psoriasis. We performed a single-center, open-label study to investigate the immunohistochemical effects in psoriatic lesional skin. A group of 11 patients with plaque psoriasis all received 12 weekly doses of 7.5 mg alefacept intravenously. Skin biopsies were obtained at baseline and on days 8, 43 and 92, and were evaluated by digital image analysis after immunohistochemical staining. After completion of treatment, 8 out of the 11 patients experienced a reduction in PASI of 50% or more compared to baseline. Immunohistochemical analysis displayed a gradual decrease in the number of cutaneous T cells during therapy, with a significant reduction in epidermal CD8+ cells and dermal CD4+ cells on day 92. Patients with a reduction in PASI of 50% or more after therapy had a clearance of effector/memory T cells from the epidermis, in contrast to patients with a reduction in PASI of less than 50%. These findings support the hypothesis that effector/memory T cells play a prominent role in the pathogenesis of psoriasis, and that alefacept is capable of reducing these cells in lesional psoriatic skin.