The purpose of this study is to help determine the effectiveness of rituxan (with or without methotrexate) in the treatment of psoriatic arthritis.

The purpose of this study is to evaluate safety and efficacy of rituximab, with and without methotrexate, in joints, enthesium and skin in psoriatic arthritis patients with inadequate response to methotrexate who have either not tried anti-TNF therapy or have had inadequate or failed response to anti-TNF therapy. To explore biologic mechanism of action via histological and immunohistochemical evaluation of pre and post treatment biopsies of psoriatic plaques.

Inclusion Criteria:

• Active disease of at least 6 months duration.
• Receiving treatment on an outpatient basis.
• The patient will have at least one evaluable skin plaque, 2 cm in diameter, that can be followed with a target lesion score (scalp and groin lesions cannot be used).
• Presence of PsA per the CASPAR categories: Psoriasis, Nail Changes, Negative RF test, Dactylitis or radiological evidence of juxta-articular new bone formation.
• Subjects will have greater than or equal to 3 tender (out of 68 joints) and 3 swollen (out of 66) joints at screening and baseline.

Exclusion Criteria:

• History of malignancy other than resolved squamous or basal cell or cervical carcinoma
• Presence of a significant medical illness that, in the opinion of the investigator, would potentially compromise the subject's ability to participate in the trial
• Presence of another rheumatic or skin disease that, in the opinion of the investigator, could confound the ability to discern response
• History or presence of HIV
• History of severe allergic or anaphylactic reactions to humanized or murine monoclonal antibodies
• History of recurrent significant infection or history of recurrent bacterial infections
• Known active bacterial, viral, fungal, mycobacterial, or other infection (including tuberculosis or atypical mycobacterial disease, but excluding fungal infections of nail beds) or any major episode of infection requiring hospitalization or treatment with i.v. antibiotics within 4 weeks of screening or oral antibiotics within 2 weeks prior to screening.
• History of psychiatric disorder that, in the judgment of the investigator, would make the patient inappropriate for entry into this trial or would lead to poor compliance.
• Concurrent treatment with any DMARD (except for MTX), any anti-TNF alpha therapy or other biologic therapy. Topical medications to treat psoriasis are limited to class VI and VII low potency steroids to the palms, soles of the feet, axilla and groin only.
• Concurrent treatment with any DMARD (except for MTX), any anti-TNF alpha therapy or other biologic therapy. Topical medications to treat psoriasis are limited to class VI and VII low potency steroids to the palms, soles of the feet, axilla and groin only.
• Treatment with any investigational agent within 4 weeks of screening or 5 half-lives of the investigational drug (whichever is longer).
• Previous treatment with any cell-depleting therapies, including investigational agents (e.g., CAMPATH, anti-CD4, anti-CD5, anti-CD3, anti-CD19).
• Previous treatment within 6 months with i.v. gamma-globulin, Orencia, toclizumab, natalizumab or Prosorba Column.
• Intra-articular or parental corticosteroid injections within 4 weeks prior to screening.
• Previous treatment with rituximab (MabThera/Rituxan)
• Immunization with a vaccine within 4 weeks prior to randomization (e.g.; MMR, Varivax, Smallpox).
• One intra-articular steroid joint injection is allowed, affected joint is excluded from assessment thereafter.
• Subjects should not take analgesics within 12 hours prior to joint assessments