Inclusion Criteria:

* Written informed consent.
* At least 16 years of age.
* Diagnosed with chronic plaque psoriasis and require systemic therapy.
* CD4+ lymphocyte counts at or above the lower limit of normal for Covance Central Laboratory Services, Inc., unless on a stable dose of prednisone (counts must be at or above 300 cells/mm3).

Exclusion Criteria:

* Unstable erthrodermic or pustular psoriasis.
* Diagnosis of guttate psoriasis.
* Serious local infection or systemic infection within 3 months prior to first dose of alefacept.
* Positive for HIV antibody.
* Known invasive malignancy within 5 years of enrollment. Patients with a history of treated squamous cell and/or basal cell carcinomas limited to the skin are not excluded.
* Evidence of active tuberculosis.
* Current treatment for active tuberculosis or tuberculosis prophylaxis.
* Female patients unwilling to practice effective contraception as defined by the investigator.
* Female patients who are pregnant or breast-feeding.
* Current enrollment in any other investigational drug study.
* Previous participation in this study or previous alefacept studies.