To evaluate the efficacy and safety of treatment with adalimumab, a fully human anti-tumor necrosis factor (anti-TNF) monoclonal antibody, over 48 weeks in patients with moderate to severe psoriatic arthritis (PsA). Patients who completed the Adalimumab Effectiveness in Psoriatic Arthritis Trial (ADEPT), a 24-week, double-blind study of adalimumab versus placebo in PsA, could elect to receive open-label adalimumab, 40 mg subcutaneously every other week after week 24. Radiographs were obtained at week 48 and were read with radiographs obtained previously. Clinical and radiographic efficacy data were analyzed overall and in patient subsets. Safety data were collected over 48 weeks. At week 48, patients from the adalimumab arm of ADEPT (n = 151) had achieved American College of Rheumatology 20% improvement (ACR20), ACR50, and ACR70 response rates of 56%, 44%, and 30%, respectively. Among those evaluated with the Psoriasis Area and Severity Index (PASI) (n = 69), PASI50, PASI75, PASI90, and PASI100 response rates (> or =50%, > or =75%, > or =90%, and 100% reduction in PASI scores, respectively) were 67%, 58%, 46%, and 33%, respectively (ACR and PASI response rates were analyzed using nonresponder imputation). Improvements in disability, as measured by the Disability Index of the Health Assessment Questionnaire (mean change in score -0.4) were sustained from week 24 to week 48. At week 24 and week 48, the mean changes from baseline in the modified total Sharp score were -0.1 and 0.1, respectively, for patients who received adalimumab for 48 weeks (n = 133), and 0.9 and 1.0, respectively, for patients who received placebo for 24 weeks followed by adalimumab for 24 weeks (n = 141). Adalimumab demonstrated clinical and radiographic efficacy regardless of whether patients were receiving methotrexate (MTX) at baseline. Adalimumab was generally safe and well tolerated through week 48. Adalimumab improved joint and skin manifestations, reduced disability, and inhibited radiographic progression over 48 weeks in patients with PsA who were participants in ADEPT. MTX use at baseline was not required for clinical or radiographic efficacy. Adalimumab had a good safety profile through week 48.