PURPOSE: Patient-reported outcome (PRO) measures are used in global clinical trials, but often initial instrument development is restricted to a single language and only subsequently translated/linguistically validated (LV) into additional target languages/countries of interest. Parallel development in multiple languages offers advantages over traditional LV methods, but also creates unique challenges. The purpose of this workshop is to present best practice and share experiences in the simultaneous development (SD) of PRO instruments in several languages. The presentations will include examples of real issues and solutions from several different SD projects. Participants will gain an overview of the simultaneous development method and its technical challenges and potential use in the overall product lifecycle. DESCRIPTION: SD of a PRO instrument in multiple languages/cultures may be a better approach for multinational instrument development, especially for culturally challenging concepts. SD will provide local language versions earlier and also helps ensure translation challenges are identified early and avoided, both for languages included in the process and for future languages. However, SD is complex and carries its own pitfalls including a lack of well-established regulatory guidelines and variation in research practices, further complicated by linguistic/cultural differences not easily understood by all stakeholders. Other challenges include coding, analyzing qualitative data consistently across languages, determining conceptual saturation across languages/cultures, ensuring item wording is conceptually equivalent, and determining if revisions in one language/culture should be implemented in all languages. These challenges may be more acute when cultures are more different (e.g. Asian vs American / European). Proposed methods, issues, and potential solutions will be explored, drawing on the presenters’ experiences in multiple SD projects and that of the scientific literature. Interactive Q&A will be conducted throughout the workshop where participants will be encouraged to share their experiences and questions with the discussants and other workshop participants.