• The proportion of patients with American College of Rheumatology (ACR) 20 response at Week 24. [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  

• The proportion of patients (with baseline = 3% body surface area (BSA) psoriatic involvement) who achieve a Psoriasis Area and Severity Index (PASI) 75 response at Week 24. [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  
• The change from baseline in total radiographic scores of the hands and feet at Week 24. [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  
• The change from baseline in the Health Assessment Questionnaire score at Week 24. [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  
• The proportion of patients with American College of Rheumatology (ACR) 50 and ACR 70 responses at Week 24. [ Time Frame: Week 24 ] [ Designated as safety issue: No ]
  

This study is a 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), parallel-group, multicenter study to evaluate the effectiveness and safety of ustekinumab compared to placebo in the treatment of patients with active psoriatic arthritis who have or are currently receiving treatment with a disease-modifying antirheumatic drug (DMARD) and/or a nonsteroidal anti-inflammatory drug (NSAID), including those who have previously received anti-tumor necrosis factor (anti-TNF) agents [(examples are infliximab (Remicade), etanercept (Enbrel), adalimumab (Humira)]. The primary effectiveness endpoint will be measured by the reduction in signs and symptoms of arthritis, as defined by 20% improvement from baseline in American College of Rheumatology (ACR) measurements of arthritis at Week 24. The study will additionally look at higher levels of joint improvement (ie, 50% or 70% improvement from baseline) and improvement in activity and quality of life, as well as the impact of ustekinumab on psoriatic skin lesions. Safety assessments will be performed throughout the study and include obtaining and evaluating laboratory tests, vital signs (eg, blood pressure) and the occurrence and severity of adverse events (side effects). Patients will be assigned to one of three treatment groups. Patients will receive either 45 mg ustekinumab, 90 mg ustekinumab, or placebo at Weeks 0, 4 and every 12 weeks until Week 40. Patients who do not have >=5% improvement in their disease (tender and swollen joints) at Week 16 may be eligible to receive an increase or change to their ustekinumab dosage. Ustekinumab 45 mg, 90 mg, or placebo subcutaneous injections at Weeks 0 and 4 followed by every-12-week dosing with the last dose at Week 40. Early escape possibility at Week 16. Patients randomized to placebo will crossover to receive ustekinumab at Weeks 24 and 28 followed by every-12-week dosing with the last dose at Week 40. Expected duration of exposure to study agent including follow up for safety is 60 weeks.