Introduction: Various factors including age, sex, body mass index (BMI) and history of operation have been linked to the colonoscopic intubation time. However, there has been few data about the factors which make the colonoscopists feel difficult colonoscopy. The aims of this study were to identify the factors associated with difficult colonoscopy. Methods: A total of 1228 consecutive subjects who underwent colonoscopy for a health checkup at a single health care center in Korea from August to December 2011 were studied prospectively. Patients' age, sex, BMI, height, waist circumference, history of operation, symptoms of irritable bowel syndrome, bowel preparation status, experience of colonoscopists, time required to reach the cecum (CIT), and the degree of difficultness which was 5 levels by the colonoscopist were analyzed. Results:. The mean age of the subjects was 51.0 ± 10.8 years, and 60.2% were male. Univariate analyses showed that experience of colonoscopists, female gender, old age, sedation, CIT, height, and prior history of surgery were associated with difficult colonoscopy. On multivariate analysis, experience of colonoscopy (OR 0.772, p=0.0002), age (OR 1.023, p=0.0001), and CIT (OR 1.007, p=1.06E-68) were related to difficult colonoscopy. Conclusion:. This study demonstrates the difficultness of insertion by colonoscopists is associated with CIT, patients' age, and experience of colonoscopists.