The purpose of this study is to evaluate the safety and efficacy (improvement of signs and symptoms) of subcutaneous (under the skin) injections of golimumab for the treatment of active psoriatic arthritis (PsA). Efficacy will be measured by reduction in the signs and symptoms of active PsA, including effects on joint pain and swelling, changes on x-ray related to joint damage, psoriasis skin lesions, physical function, and quality of life.

Anti-tumor necrosis factor (TNF) agents have been shown to be effective in improving arthritis and psoriasis symptoms in patients with active psoriatic arthritis. Golimumab is a new anti-TNFα agent. This is a multicenter, randomized (patients are assigned different treatments based on chance), double-blind (neither the patient nor the physician knows whether drug or placebo is being taken, or at what dosage), placebo-controlled, parallel group study comparing safety and efficacy of golimumab 50mg, golimumab 100mg, and placebo subcutaneous injections administered every 4 weeks, in subjects with active PsA. The total duration of treatment is approximately 5 years. In the first portion of the study, some patients will be randomly assigned to receive placebo treatment through the Week 20 injection; others will be assigned to golimumab 50mg or golimumab 100mg groups through the Week 20 injection. There is an "early escape" at Week 16 in the study whereby patients who meet criteria for minimal improvement in their joints will be switched to golimumab if they were on placebo, or have the golimumab dose increased if they were originally assigned to the golimumab 50mg group. At Week 24, the placebo group subjects will switch to golimumab 50mg injections, and all patients will continue receiving in a blinded manner either 50 or 100mg golimumab injections every 4 weeks until the first 52 weeks of data are fully collected on all the subjects (database lock). After this 52-week database lock, everyone will be unblinded to the golimumab dose, and continue to receive golimumab treatment through Week 252 as part of a long-term extension phase of the study, with options for adjusting concomitant PsA medications and/or increasing the dose of golimumab. The study hypothesis is that golimumab will be more effective than placebo both in terms of reducing the signs and symptoms of PsA, as measured by the American College of Rheumatology (ACR) 20 response at Week 14, and inhibiting the amount of damage due to PsA seen on x-rays of the hand and feet at Week 24, while maintaining an acceptable safety profile.

Golimumab 50mg, Golimumab 100mg, or placebo injected under the skin every 4 weeks at Weeks 0, 4, 8, 12, 16, and 20, followed by injections of either Golimumab 50mg or Golimumab 100mg every 4 weeks, for approximately 5 years total duration from the time of the first study agent injection.

• Biological: golimumab
• Biological: Placebo; golimumab

Inclusion Criteria:

• Psoriatic arthritis (PsA) diagnosed > 6months prior
• Active PsA at the time of screening and at baseline visits, with >= 3 swollen joints and >= 3 tender joints
• Have at least 1 of the PsA subsets (DIP joint arthritis, polyarticular arthritis without rheumatoid nodules, arthritis mutilans, asymmetric peripheral arthritis, or spondylitis with peripheral arthritis)
• Active plaque psoriasis with a lesion >= 2cm in diameter
• Active arthritis despite current disease modifying anti-rheumatic drug (DMARD) or nonsteroidal anti-inflammatory drug (NSAID) therapy
• Stable doses of methotrexate, low-dose corticosteroids, and NSAIDs are permitted

Exclusion Criteria:

• No prior treatment with biologic anti-TNF agents (infliximab, etanercept, adalimumab)
• No treatment with alefacept or efalizumab within 3 months prior to the first study drug injection
• No DMARDs other than methotrexate, or immunosuppressive drugs within 4 weeks prior to the first study drug injection