Developing new patient-reported outcome measures (PROMs) for application in clinical studies can be necessary if an adequate PROM does not exist. For adequate measurement, it is essential that the PROM has face validity (ie, is perceived to be relevant by clinicians and researchers) and has high content validity (ie, content relevance and content coverage for the targeted patient group). The steps needed to create PROMs that possess face and content validity for a specific condition are described in this paper. Face validity is achieved by item identification and generation through literature review. Content validity is confirmed through repetitive cognitive interviews of patients from the targeted patient group in order to generate a consensus-based pilot-version of the new PROM. This qualitative process ensures that items are appropriately worded, understandable, and minimizes doubts about how items should be answered. A practical example of this process is presented, which shows the development of the Knee Numeric-Entity Evaluation Score (KNEES-ACL), a condition-specific PROM for patients with deficiency of the anterior cruciate ligament (ACL).