Background <p>Because of the nature of colonoscopy, missed colon adenoma occurs, of which flat adenoma is closely related to the interval cancer. Thus, methods to reduce missed adenoma have been studied but are still lacking.</p> Aims <p>To investigate the relationship between the withdrawal time of the proximal colon and the missed flat adenoma, and to find the appropriate withdrawal time.</p> Methods <p>We retrospectively studied the medical records of patients who underwent screening colonoscopy. According to the proximal colon withdrawal time, the patients were classified into two groups and compared with the clinical characteristics, adenoma detection rate (ADR), and flat adenoma detection rate (FADR). The correlation between adenoma miss rate (AMR), flat adenoma miss rate (FAMR), and withdrawal time of proximal colon was analyzed.</p> Results <p>A total of 10,253 patients were enrolled, of whom 1273 received colonoscopy more than twice. Based on proximal colon withdrawal time, 40.6% (<i>n</i> = 4162) were less than 3&#xa0;min and 59.4% (<i>n</i> = 6091) were more than 3&#xa0;min. Compared to a group with a withdrawal time of less than 3&#xa0;min, the group with a withdrawal time of more than 3&#xa0;min had a significantly high proximal colon ADR (17.8% vs. 21.1%, <i>P</i> = 0.000) and FADR (4.2% vs. 5.4%, <i>P</i> = 0.006), and statistically low AMR (17.9% vs. 12.8%, <i>P</i> = 0.016) and FAMR (8.2% vs. 4.0%, <i>P</i> = 0.003). Multivariate analysis also showed that the group with a withdrawal time longer than 3&#xa0;min had a significantly low AMR (OR 1.381; 95% CI 1.025–2.012, <i>P</i> = 0.043) and FAMR (OR 2.053; 95% CI 1.198–3.417, <i>P</i> = 0.018).</p> Conclusions <p>Flat adenoma detection and miss rate of the proximal colon were significantly different according to the withdrawal time of 3&#xa0;min. Proximal colon withdrawal time of more than 3&#xa0;min is required for colonoscopy, and it is expected to reduce interval cancer.</p>