Psoriatic arthritis (PsA) is an inflammatory arthritis associated with psoriasis, which has a estimated prevalence of 0.3 - 1 %. The clinical course varies, but PsA is often a progressive, erosive arthritis causing severe disability and increased mortality.||The biologic treatment infliximab and etanercept have recently been introduced for treatment of PsA and psoriasis, and current data indicate a higher efficacy than with previously available therapies. No clinical trials on adalimumab in PsA are yet published (2005), but preliminary data are encouraging. The improved treatment options have increased the need for sensitive methods for diagnosis, monitoring and prognostication of PsA, so that the efficient therapies can be initiated at the optimal time point and monitored optimally. Ultrasonography (US) and magnetic resonance imaging (MRI) and a number of biomarkers are promising, but not yet sufficiently studied, methods for this.||The hypothesis is that adalimumab will be an effective treatment option for PsA. Novel imaging- and biomarkers can provide additional information, compared to clinical measures and radiography, concerning activity, destructive progression and prediction of therapeutic response in PsA patients receiving adalimumab. The perspective is a potential improvement in diagnosis, monitoring and prognostication of patients with PsA.