 Patients with moderate to severe psoriasis will be included in this multicenter, double-blind, placebo controlled study. Patients will be randomized 1:1 at Day 0 to receive either adalimumab or placebo. Adalimumab will be administered sub-cutaneously as described in the Canadian product monograph 80mg followed by 40mg at Week 1 and 40mg every other week. At Week 16, all patients will receive two injections of blinded study products. Patients randomized to the placebo group will receive two injections of adalimumab 2 x 40 mg and patients randomized to adalimumab will receive two injections of placebo. As of Week 17, patients randomized to the placebo group will receive 40 mg adalimumab every other week until Week 67. Patients randomized to the adalimumab group will continue to receive adalimumab 40mg every other week until Week 51. 