The purpose of the study is to assess the safety and efficacy of ABT-874 in the treatment of moderate to severe chronic plaque psoriasis.

Inclusion Criteria:

• Subject was age 18 or older and in good health (Investigator discretion) with a recent stable medical history.
• Subject had a clinical diagnosis of psoriasis for at least 6 months, and had moderate to severe plaque psoriasis

Exclusion Criteria:

• Subject had previously received systemic or biologic anti-IL-12 therapy
• Subject cannot discontinue systemic therapies and/or topical therapies for the treatment of psoriasis and cannot avoid UVB or PUVA phototherapy
• Subject is taking or requires oral or injectable corticosteroids
• Subject considered by the investigator, for any reason, to be an unsuitable candidate for the study
• Female subject who is pregnant or breast-feeding or considering becoming pregnant