The consequences of advocacy in nursing are critical when caring for a potential organ donor. No specific instrument has been available to measure attitudes toward organ donor advocacy. The aim of this study was to develop and psychometrically evaluate an instrument for measuring intensive and critical care (ICU) nurses' attitudes toward organ donor advocacy. The study was conducted in two stages: instrument development and instrument evaluation and refinement. A questionnaire was developed (Attitude Toward Organ Donor Advocacy Scale (ATODAS)), which was sent to half of all nurses working in ICUs (general-, neuro-, thoracic- or paediatric-) in Sweden (n=1180). The final response rate was 42.5% (n=502). In order to explore validity and reliability, the expected scale dimensionality of the questionnaire was examined both by explorative principal component analysis (with oblique, varimax rotation) and by confirmatory multi-trait analysis. The confirmatory factor analysis indicated that the ATODAS could best be explained by five factors; Attitudes toward championing organ donation at a structural hospital level; Attitudes toward championing organ donation at a political and research level; Attitudes toward actively and personally safeguarding the will and wishes of the potential organ donor, Attitudes toward safeguarding the potential donor's will and wishes by a professional approach and Attitudes toward safeguarding the will and wishes of the relatives. This initial testing indicated that the ATODAS has good psychometric properties and can be used in future research to explore if interventions may influence attitudes and behaviors related to organ donor advocacy.