Background: There is a need for data on children's self-reported discomfort in clinical research, helping ethics committees to make their evaluation of discomfort described in study protocols evidence-based. Since there is no appropriate instrument to measure children's discomfort during medical research procedures, we aimed to develop a generic, short and child-friendly instrument: the DISCO-RC questionnaire (DISCOmfort in Research with Children). Methods: This article describes the six steps of the development of the DISCO-RC. First, we updated a literature search on children's self-reported discomfort in clinical research to get insight in what words are used to measure discomfort (step 1). Subsequently, we interviewed 46 children (6-18years) participating in research to get insight into important forms of discomfort for children (step 2), and asked them about their preferred response option for measuring discomfort (step 3). Next, we consulted nine paediatric research professionals from various backgrounds for input on the content and feasibility of the DISCO-RC (step 4). Based on the previous steps, we developed a draft version of the DISCO-RC, which we discussed with the professionals. The DISCO-RC was then pretested in 25 children to ensure face-validity from the child's perspective and feasibility (step 5). Finally, validity, reliability and internal consistency were tested (step 6). Results: The search-update revealed several words used for measuring discomfort in research (e.g. 'worries', 'unpleasantness'). The interviews gave insight into important forms of discomfort for children in research (e.g. 'pain', 'boredom'). Children preferred a 5-point Likert scale as response option for the DISCO-RC. The experts recommended a short, digital instrument involving different forms of discomfort, and measuring discomfort of individual research procedures. Pretesting of the DISCO-RC resulted in a few layout changes, and feedback from the children confirmed the feasibility of the DISCO-RC. Convergent validity and test-retest reliability were acceptable. Internal consistency based on item-rest correlations and Cronbach's alpha were low, as expected. Conclusions: The DISCO-RC is a generic, practical and psychometrically sound instrument for measuring children's discomfort during research procedures. It contributes to make the evaluation of discomfort in paediatric research evidence-based. Therefore, we recommend including the DISCO-RC as standard component of paediatric research studies.