Background: Symptom-checker self-triage apps assist patients to determine the urgency of medical care. To be safe and effective, these tools must be validated, particularly to avoid potential hazardous undertriage.Objective: To investigate the safety of patient's self-triage using a symptom-checker.Methods: A single centre, prospective clinical trial comparing the individual outcomes of patients' self-triage with the assessment of the clinical urgency made by three successive interdisciplinary panels of physicians. Data collected between 25 November 2019 and 1 May 2020. Panel assessments and data analysis completed on 29 August 2022. Setting: Walk-in-Clinic and Interdisciplinary Emergency Department (WIC/ED) of the cantonal hospital of Baden, Switzerland. Partcipants: All patients ? 18 years attending the WIC/ED between 8:00 a.m. and 5:00 p.m. Exclusion criteria included ESI 1 and presence of symptoms not encompassed by the symptom-checker. 22 676 (12 655 ER, 10 021 WIC) used the facility during the enrollment period. 7550 patients attended WIC/ED within the recruitment window. 2550 gave informed consent, 7 patients withdrew. Intervention: Participants assessed their symptoms using SMASSPathfinder, a web-based symptom-checker based on a computerized transparent neural network. Main Outcome and Measure: The assessment by the panels encompassed the appropriate time-to-treat and the adequate point-ofcare. If a case was adjudicated as undertriaged by the first two panels, the third panel assessed the patient's risk to health or life, making a decision on whether a potentially hazardous undertriage had been present. Using a Clopper-Pearson confidence interval, we assumed that in order to confirm the symptom-checkers safety, the upper confidence bound should lie below 1%.