The efficacy and toxicity of immunosuppressive therapy using the adenine analogue fludarabine will be evaluated in 20 patients with psoriatic arthritis, who have failed or have developed intolerable side-effects to at least one disease modifying antirheumatic drug including sulfasalazine, gold, methoxypsoralen and long wave ultraviolet A light (PUVA), retinoids, methotrexate or cyclosporin. In this double-blind, placebo-controlled trial patients will receive a four month course of intravenous fludarabine (30 mg/m(2)/day for 2-3 days every 4 weeks for a total of four cycles) or placebo by a block randomization procedure to ensure groups balanced for disease activity. After a washout period of two months, the patients receiving placebo will have the option of crossing over to the fludarabine arm for an additional four months of treatment. Disease activity (both skin and joints) will be monitored throughout the study. At the end of the study, physician and patient assessment of disease activity as well as drug-related toxicities will be analyzed.