BACKGROUND/AIMS: The influence of the endoscopist on the polyp detection rate (PDR) is underappreciated in clinical practice. Moreover, flat lesions or lesions of the proximal colon are more difficult to detect. Here, we evaluated the differences in the PDR and the characteristics of detected polyps according to the experience of the colonoscopist. METHODS: We collected data on 2,549 patients who underwent screening colonoscopy performed by three fellows. The PDR was calculated according to the percentage of patients who had at least one polyp (method A) and according to the percentage of detected lesions (method B). The primary outcome included the change in the PDR, and the secondary outcome included the change in the characteristics of the detected polyps with increasing experience of the colonoscopist. RESULTS: No proportional correlation was found between the PDR and increasing experience in colonoscopy with method A; however, with method B, the PDR increased after 400 colonoscopies (p=0.0209). With method B, the detection rates of small polyps (<5 mm) (p=0.0015) and polyps in proximal sites (p=0.0050) increased after 300 colonoscopies. CONCLUSIONS: Our study demonstrated that the quality of a colonoscopy, measured by using the PDR, may increase when performed by experienced fellows.