Background: Smaller guiding catheters may reduce patient���s discomfort during transradial coronary intervention (TRI). This study was to see whether 5-French (F) guiding catheter approach can reduce patient���s discomfort compared to larger guiding catheter (6-french) during TRI. Methods: This study is a single-centre prospective randomised study to compare the success rate and patient���s morbidity during TRI between 5-F (99 patients, 120 lesions) and 6-F (97 patients, 122 lesions) guiding catheter approach. The patient discomfort score (PDS) was defined into 6 grades (0:none, 1:mild discomfort, 2:considerable discomfort, 3:painful, 4:very painful, 5:intolerable) during sheath insertion, interventional procedure, sheath removal, and 24 hours after the procedure. Before the procedure, mean radial artery diameter was measured by radial angiography after nitroglycerin injection. Results: The mean age of patients was 62.1짹 10.0 years and 70.9% of patients were male. The procedural success rate was similar between 2 groups (5-F: 99.2%, 6-F 98.4%). Only one of 5-F group was crossed over to 6-F group. Guiding support was satisfactory in both groups (5-F 93.9%, 6-F 96.9%, p=0.32). The incidences of local complications such as hematoma, arterial occlusion, the difficulty in sheath removal were not significantly different between two groups. 5-F group was associated with significantly lower PDS compared to 6-F group during interventional procedure (0.19짹0.06 vs. 0.61짹0.09, p<0.001) and 24hr after the procedure (0.22짹0.06 vs 0.43짹0.09, p<0.001). The number of patients with PDS 쨀3 was significantly lower in 5-F group during sheath insertion (5-F 5.1%, 6-F 24.7%, p<0.001), and during sheath removal (5-F 7.1%, 6-F 38.1%, p<0.001). The sheath-to-artery diameter ratio was significantly lower in 5-F group (5-F 0.78짹0.02, 6-F 0.85짹0.02, p=0.003). Conclusions: This study suggests that TRI using a 5-F guiding catheter approach may be associated with less patient���s discomfort and similar success rate compared to 6-F approach.