AIM: to compare the feasibility and patients’ tolerance of esophagogastroduodenoscopy (EGD) using a thin endoscope with those of conventional oral EGD and to determine the optimal route of introduction of small-caliber endoscopes. METHODS: One hundred and sixty outpatients referred for diagnostic EGD were randomly allocated to 3 groups: conventional (C)-EGD (9.8 mm in diameter), transnasal (TN)-EGD and transoral (TO)-EGD (5.9 mm in diameter). Pre-EGD anxiety was measured using a 100-mm visual analogue scale (VAS). After EGD, patients and endoscopists completed a questionnaire on the pain, nausea, choking, overall discomfort, and quality of the examination either using VAS or answering some questions. The duration of EGD was timed. Blood oxygen saturation (SaO2) and heart rate (HR) were monitored during EGD. RESULTS: Twenty-one patients refused to participate in the study. The 3 groups were well-matched for age, gender, experience with EGD, and anxiety. EGD was completed in 91.1% (41/45), 97.5% (40/41), and 96.2% (51/53) of cases in TN-EGD, TO-EGD, and C-EGD groups, respectively. TN-EGD lasted longer (3.11 ± 1.60 min) than TO-EGD (2.25 ± 1.45 min) and C-EGD (2.49 ± 1.64 min) (P < 0.05). The overall tolerance was higher (P < 0.05) and the overall discomfort was lower (P < 0.05) in TN-EGD group than in C-EGD group. EGD was tolerated “better than expected” in 73.2% of patients in TN-EGD group and 55% and 39.2% of patients in TO-EGD and C-EGD groups, respectively (P < 0.05). Endoscopy was tolerated “worst than expected” in 4.9% of patients in TN-EGD group and 17.5% and 23.5% of patients in TO-EGD and C-EGD groups, respectively (P < 0.05). TN-EGDcaused mild epistaxis in one case. The ability to insufflate air, wash the lens, and suction of the thin endoscope were lower than those of conventional instrument (P < 0.001). All biopsies performed were adequate for histological assessment. CONCLUSION: Diagnostic TN-EGD is better tolerated than C-EGD. Narrow-diameter endoscope has a level of diagnostic accuracy comparable to that of conventional gastroscope, even though some technical characteristics of these instruments should be improved. Transnasal EGD with narrow-diameter endoscope should be proposed to all patients undergoing diagnostic EGD. AIM: to compare the feasibility and patients’ tolerance of esophagogastroduodenoscopy (EGD) using a thin endoscope with those of conventional oral EGD and to determine the optimal route of introduction of small-caliber endoscopes. METHODS: One hundred and sixty outpatients referred for diagnostic EGD Pre-EGD anxiety was measured using a 100- After EGD, patients and endoscopists completed a questionnaire on the pain, nausea, choking, overall discomfort, and quality of the examination either using VAS or answering some questions. The duration of EGD was timed. Blood oxygen The three groups were well-matched for age, gender, experience with EGD, and anxiety. EGD was completed in 91.1% (41/45 (51/53) of cases in TN-EGD, TO-EGD and C-EGD groups, respectively. TN-EGD lasted longer (3.11 ± 1.60 min) than TO The overall tolerance was higher (P <0.05) and the overall discomfort was lower (P <0.05) in the TN-EGD group (2.25 ± 1.45 min) and C-EGD than in C-EGD group. EGD was tolerated “better than expected ” in 73.2% of patients in TN-EGD group and 55% and 39.2% of patients in TO-EGD and C-EGD groups, respectively ). Endoscopy was tolerated “worst than expected ” in 4.9% of patients in TN-EGD group and 17.5% and 23.5% of patients in TO-EGD and C-EGD groups, respectively (P <0.05) Mild epistaxis in one case. The ability to insufflate air, wash the lens, and suction of the thin endoscope were lower than those of conventional instruments (P <0.001). All biopsies performed were for histological assessment. CONCLUSION: Diagnostic TN-EGD is better tolerated than C-EGD. Narrow-diameter endo scope has a level of diagnostic accuracy comparable to that of conventional gastroscope, even though some technical characteristics of these instruments should be improved. Transnasal EGD with narrow-diameter endoscope should be proposed to all patients undergoing diagnostic EGD.