Several studies have documented an essential role for interleukin-10 (IL-10) in preventing prolonged and exaggerated immune responses to antigens and irritants. Psoriasis, a relatively common disease, is characterized by T cell-mediated inflammation in affected skin. In this study, the safety, tolerance, immunologic effects, and clinical activity of subcutaneous (SC) recombinant human (rh) IL-10 will be evaluated in patients with moderate-to-severe psoriasis. There will be 2 groups of patients, randomized to receive either 20 (micro)g/kg rhIL-10 SC 3 times weekly (20 patients) or SC placebo (10 patients). This double-blind phase will continue for a total of 12 weeks and the principal evaluation will be the comparison between baseline and 12 week Psoriasis Area Severity Index (PASI) scores. Patients will come for an initial screening visit at day 0, and at weeks 1, 2, 4, 6, 8, and 12, with follow-up visits at weeks 16 and 20.