Medical history, physical examination, blood pressure and heart rate, and ECGs are checked periodically. Blood samples will be taken for standard laboratory tests as well as special tests of the drug blood level, biomarkers to see whether the drug might be affecting the biological pathway of antigen presentation, and skin biopsies from areas both involved and not involved with psoriasis. The blood and skin samples will be used to see whether some of the characteristics of psoriasis are changed by taking the drug. Samples will also be taken to see whether certain types of immune cells are affected by taking the drug. Immunizations with tetanus and hepatitis A vaccines will be done to see if taking the drug might affect immune responses to these agents. Patients will receive RWJ-445380, 50, 100, 200, 300 mg, or placebo. Patients will receive an oral capsule daily for up to 12 weeks.