BACKGROUND AND STUDY AIMS Upper gastrointestinal endoscopy is performed without sedation in many countries. Unsedated patients experience more discomfort during endoscopy than sedated patients, but few studies have examined factors which could be modified to minimize discomfort during the procedure. We assessed the effect of endoscope diameter on patient discomfort during unsedated transoral gastroscopy. PATIENTS AND METHODS A total of 322 patients attending for unsedated endoscopy were examined using an endoscope of diameter either 6.0 mm or 9.8 mm. Patients completed a two-part questionnaire assessing tolerance of the procedure and discomfort during it. RESULTS There was failure to complete the initial unsedated endoscopy in three of 163 patients in the 6.0 mm group and 14 of 159 in the 9.8 mm group (P = 0.009). Patients in the 6.0 mm group reported less discomfort both during endoscope insertion (P < 0.0001) and during the remainder of the procedure (P < 0.0001). 14% of patients in the 6.0 mm group indicated that they would request sedation if a further endoscopy were necessary, compared with 31% in the 9.8 mm group (P = 0.0005). CONCLUSIONS Ultrathin endoscopes may have a role in clinical practice if randomized comparative studies with standard-bore instruments confirm that they do not compromise diagnostic quality.