• Proportion of patients achieving a ≥ 75% decrease in the PSSI [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
• Proportion of patients achieving a ≥ 50% decrease in the PSSI [ Time Frame: Weeks 12 and 24 ] [ Designated as safety issue: No ]
• Proportion of patients who achieve a whole body (including the scalp) PGA rating of clear (0), almost clear (1), or mild (2) [ Time Frame: Weeks 12 and 24 ] [ Designated as safety issue: No ]
• Change in the Scalpdex (a quality-of-life instrument for scalp dermatitis) [ Time Frame: Weeks 12 and 24 ] [ Designated as safety issue: No ]
• Change in a VAS of patient reported scalp itch [ Time Frame: Weeks 12 and 24 ] [ Designated as safety issue: No ]
• Change in BSA affected by psoriasis on the entire body [ Time Frame: Weeks 12 and 24 ] [ Designated as safety issue: No ]

This is a Phase IV randomized, double-blind, placebo-controlled study to evaluate the safety and efficacy of subcutaneous efalizumab in adult patients (18 years of age and older) with chronic moderate to severe plaque psoriasis with involvement of the scalp who have had no previous exposure to efalizumab. The study will consist of a screening period, a double-blind treatment period, an open-label treatment period, and an observation/follow-up period. The planned enrollment for the trial is 100 patients.

• Drug: efalizumab
  Subcutaneous repeating dose
• Drug: placebo
  Subcutaneous repeating dose

• 1: Experimental
  Intervention: Drug: efalizumab
• 2: Placebo Comparator
  Intervention: Drug: placebo

Inclusion Criteria:

• Provide written informed consent and Health Insurance Portability and Accountability Act of 1996 (HIPAA) documents
• Be aged 18 years or older
• Have chronic (6 months or greater) moderate to severe plaque psoriasis with involvement of the scalp
• Have a whole body PGA rating of moderate (3) or severe (4) for psoriasis
• Have a scalp surface area (SSA) affected by plaque psoriasis of 30% or more, assessed using PSSI
• Have at least one of three clinical signs involving the scalp (erythema, induration, and desquamation) be rated as at least moderate (2) with the other two rated as at least slight (1) using PSSI
• Be a candidate for systemic therapy in the opinion of the investigator
• Be naive to efalizumab treatment
• For women of childbearing potential, must be willing to use a method of contraception acceptable to the investigator to prevent pregnancy and agree to continue to practice an acceptable method of contraception for the duration of their participation in the study and for 6 weeks after the last dose of efalizumab

Exclusion Criteria:

• Have had previous exposure to efalizumab or a history of hypersensitivity to any of its components
• Are using any excluded therapy
• Have a history of drug or alcohol abuse in the past five years
• Have a history in the past 5 years of a serious infection or currently have an ongoing uncontrolled bacterial, viral, fungal, or atypical mycobacterial infection
• Have any history of opportunistic infections (e.g., systemic fungal infections, parasites)
• Are seropositive for hepatitis B antigen, hepatitis C antibody, or human immunodeficiency virus (HIV)
• Have a history of active tuberculosis or are currently undergoing treatment for tuberculosis, including latent tuberculosis with isonazid
• Have the presence or history of malignancy within the past 5 years, including lymphoproliferative disorders
• Are pregnant or breastfeeding
• Have a diagnosis of hepatic cirrhosis, regardless of cause or severity
• Have a history in the last 5 years of thrombocytopenia
• Have a history in the last 5 years of hemolytic anemia or any other clinically significant anemia
• Have been exposed to any experimental and/or unapproved drugs or treatments within 30 days or 5 half-lives, whichever is longer, before the screening visit
• Have been vaccinated with a live virus or live bacteria within the 14 days before the first dose of efalizumab
• Have any medical condition that, in the judgment of the investigator, would jeopardize the patient's safety following exposure to study drug (efalizumab or placebo equivalent) or would significantly interfere with the patient's ability to comply with the provisions of this protocol