In a multicenter cross-sectional study, 73 adult patients treated with adalimumab (40 mg) every other week for at least 24 weeks are assessed for psoriasis disease severity through measurement of the Psoriasis Area and Severity Index (PASI) before adalimumab treatment start, and prior to sampling. Patients who interrupted their treatment schedule during the 24 weeks prior to blood sampling are excluded. Samples of patients who were treated with adalimumab for any other inflammatory disease and later developed psoriasis are also excluded. Percentage of PASI improvement compared to baseline (∆PASI) represents clinical response. Patients are classified as nonresponders (∆PASI<50), moderate responders (∆PASI 50-75) or good responders (∆PASI 75-100).Serum is collected for adalimumab trough level and anti-drug antibody determination (Sanquin, The Netherlands). By receiver-operator characteristics (ROC) analysis, a cut-off value of adalimumab trough level can be determined to distinguish insufficient from adequate responders.