Background: Although it is known that right-side colon cancers increase with aging, the location of adenomas according to age has been controversial. Adenomas found at initial colonoscopies may be a mixture of polyps that arose at various earlier ages. Objectives: To elucidate the relationship between location and age at which adenomas actually developed. Design: Prospective cohort study. A large-scale health appraisal institution in Japan. Patients: A total of 23,444 consecutive, asymptomatic Japanese who underwent total colonoscopy at their annual medical health check-up. Interventions: We analyzed newly developed adenomas after confirmation of the Absence of colorectal neoplasms by two serial total colonoscopies. Main Outcome Measurements: The results of follow-up colonoscopies in 6304 subjects with no colorectal neoplasms at 2 initial colonoscopies were analyzed. The locations were separated into right-side colon and left-side colon from the splenic flexure. The locations of new adenomas found for the first time during the follow-up term were analyzed. Results: A total of 1472 subjects developed new colorectal adenomas. In 1255 cases with single adenomas, the ratios of number of cases with right-side lesions to that with left-side lesions at the ages of < 40 years, 40 to 49 years, 50 to 59 years, 60 to 69 years, and ≥ 70 years were 1.18, 1.00, 1.29, 1.31, and 1.89, respectively. In 217 cases with multiple adenomas found concurrently, the percentage of cases with adenomas only on the right side at the ages of < 40 years, 40 to 49 years, 50 to 59 years, 60 to 69 years, and ≥ 70 years were 0% , 25% , 34% , 37% , and 63% , respectively. Limitations: Small lesions might have been missed even by 2 serial colonoscopies. Conclusions: Adenomas on the right-side colon increased with aging. Background: Although it is known that right-side colon cancers increase with aging, the location of adenomas according to age has been controversial. Adenomas found at initial colonoscopies may be a mixture of polyps that arose at various earlier ages. Objectives: To elucidate the Relationship between location and age at which adenomas actually developed. Design: Prospective cohort study. A large-scale health appraisal institution in Japan. Patients: A total of 23,444 consecutive, asymptomatic Japanese who underwent total colonoscopy at their annual medical health check-up. Interventions: We analyzed newly developed adenomas after confirmation of the Absence of colorectal neoplasms by two serial total colonoscopies. Main Outcome Measurements: The results of follow-up colonoscopies in 6304 subjects with no colorectal neoplasms at 2 initial colonoscopies were analyzed. The locations were separated into right-side colon and left-side colon from the splenic flexure. The locations of new ad Results: A total of 1472 subjects developed new colorectal adenomas. In 1255 cases with single adenomas, the ratios of number of cases with right-side lesions to that with left- side lesions at the ages of <40 years, 40 to 49 years, 50 to 59 years, 60 to 69 years, and> 70 years were 1.18, 1.00, 1.29, 1.31, and 1.89, respectively. concurrently, the percentage of cases with adenomas only on the right side at the ages of <40 years, 40 to 49 years, 50 to 59 years, 60 to 69 years, and ≥ 70 years were 0%, 25%, 34% 37%, and 63%, respectively. Limitations: Small lesions might have been missed even by 2 serial colonoscopies. Conclusions: Adenomas on the right-side colon increased with aging.