OBJECTIVES: Although there are a range of established outcomes measures for psoriasis in general, there has been a lack of valid instruments for nail psoriasis (NPso). Therefore, we developed and validated the three-component tool “Nail Assessment in Psoriasis and Psoriatic Arthritis” (NAPPA) including health-related quality of life (NAPPA-QoL), patient-relevant treatment benefit (NAPPA-PBI) and clinical severity (NAPPA-CLIN). METHODS: The NAPPA tool was developed by a multinational expert group involving dermatologists, psychologists, statisticians, and patients. Development included the following steps: 1) Open item collection on patient-relevant impairments and needs in n=120 patients in 2 countries (D, USA); 2) item development by an expert panel including patients; 3) double forward and backward translations with subsequent translators' and developers’ conferences; 4) feasibility testing of the pilot questionnaire in n=55 patients in 4 countries (D, USA, Canada, UK) with subsequent questionnaire refinement; 5) longitudinal feasibility and validation study in n=203 patients from 6 countries (Germany, USA, Denmark, Japan, Italy, Spain). RESULTS: Open item collection generated 692 single items with redundant content which could be condensed to 20 items for the NAPPA-QoL and 24 items for the NAPPA-PBI. Most patients rated the final questionnaires as feasible, i.e. the purpose was clear to them (95.0%), instructions and questions were comprehensible (83.6% / 89.1%), and all relevant areas were covered (87.1%). NAPPA-QoL and -PBI correlated moderately with clinical outcomes (PASI, NAPSI) but markedly with other QoL questionnaires (EQ-5D, DLQI). Sensitivity to change and internal consistency were good. Clinical severity (NAPPA-CLIN) was measured with a two-digit solution which correlated highly (r>0.9) with NAPSI but which can be assessed much more time-efficiently. CONCLUSIONS: With the modular NAPPA outcomes tool, clinical and patient-reported outcomes in nail psoriasis can be measured validly and reliably. Thus, it can be recommended for usage in international clinical studies and daily practice.