Pathologic T-cell activation is implicated in psoriasis progression. CD80, a costimulatory molecule involved in T-cell activation, likely plays a key role. IDEC-114, an IgG₁ anti-CD80 antibody, was evaluated for safety, pharmacokinetics, and preliminary clinical activity in this open-label, single-dose, dose-escalating study in patients with moderate to severe chronic plaque psoriasis. Twenty-four patients received IDEC-114 (0.05 mg/kg, 0.25 mg/kg, 1 mg/kg, 5 mg/kg, 10 mg/kg, or 15 mg/kg). Psoriasis Area and Severity Index, Physician's Global Psoriasis Assessment, and Psoriasis Severity Scale scores improved in the highest-dose groups. Average plaque thickness and plaque CD3⁺ and CD8⁺ T-cell counts decreased in the 10 mg/kg dose group. Adverse events were primarily mild, transient, constitutional symptoms; the most common related events were mild asthenia (29% of patients), chills (25%), and headache (21%). The serum half-life of IDEC-114 was approximately 13 days. A single dose of IDEC-114 appears to be safe and well tolerated and has promising clinical activity in psoriasis. (J Am Acad Dermatol 2002;47:692-700.)