The exact prevalence of psoriatic arthritis (PsA) among patients with psoriasis is still not conclusive. Data in the literature vary between 5.8 and 30 %. Objective of this study was to gain more information on the prevalence of PsA among patients with psoriasis in Germany. Between 09/2010 and 05/2011, consecutive patients from dermatological private practices and a university hospital with psoriasis were asked to fill out the validated German Psoriatic Arthritis Diagnostic (GEPARD) Questionnaire. Patients who answered ≥4 questions with "yes" were invited to come for a rheumatological check up. Those patients who refused a rheumatological examination were counted as "absence of PsA". Laboratory tests for inflammatory markers as well as the severity of skin manifestations were assessed. The diagnosis of PsA was made according to the CASPAR criteria, and imaging was performed in addition. A total of 404 questionnaires were evaluated; 50.5 % answered ≥4 questions positively; 19.3 % had a history of PsA confirmed by a rheumatologist; and in 10.9 %, PsA or spondyloarthritis was newly diagnosed during the present study. This leads to an overall prevalence of PsA in patients with psoriasis of 30.2 %. The frequency of psoriatic arthritis in the present study is higher than expected from previous studies in Germany. The prevalence is consistent with findings of a large observational survey from Scandinavia. Using the CASPAR criteria and imaging in all patients, certainty of the diagnosis is very high. The GEPARD Questionnaire is a helpful tool to identify people at risk for psoriatic arthritis.