OBJECTIVES: Photo-based questionnaires have been used to evaluate aesthetic treatment outcomes such as wrinkle reduction and eyelash growth.  The objective of this study was to develop a photonumeric guide of scar severity intended for use by both clinicians and patients to measure scar treatment outcomes. METHODS: Nine clinicians and 43 patients with linear surgical scars participated in a photograph sorting exercise.  Patients sorted approximately 50 scar photographs consistent with their own skin type (light, medium or dark skin), while clinicians sorted all three sets of photographs (n=151 photographs).  All participants arranged the photographs into 5 categories of perceived scar severity (least to most severe) and 5 photographs were considered for inclusion in the photonumeric guide based on analysis of inter-rater reliability, response consistency, redundancy, and variability. When photographs yielded similar results, clinical judgment was used to select the best photo. Instruction and response anchors were developed for the 5 scar photographs and the final guide was cognitively debriefed for relevance, comprehensibility, and acceptability in 24 additional scar patients. RESULTS: Based on the pre-specified criteria for inclusion and exclusion, 5 light skin and 5 dark skin photographs were included in the final photonumeric guide.  A “medium” guide was not developed because of significant overlap between it and the light skin guide rendering it superfluous. Inter-rater reliability of the 52 subject cohort was strong (0.95-0.96) across all skin types. The 5 photographs included in the photonumeric guide demonstrated goodness-of-fit (infit mean-square < 1.4 and > 0.6), low variance in severity ratings (SD < one category change), and strong agreement between patients and clinicians. CONCLUSIONS: The Patient and Clinician Reported Scar Severity Scales were systematically developed to easily assess scar severity outcomes in clinical trials. Continued psychometric evaluation of the guide is planned to ensure the scales meet regulatory standards for labeling purposes.