BACKGROUND/AIMS: Amoung patients with diminutive (1-9 mm in diameter) adenomatous polyps (DAP) found during flexible sigmoidoscopy (FS), the likelihood of identifying advanced adenomas (i.e., adenomas ≥ one cm in diameter, tubulo-villous or villous adenomas, and adenomas with high grade dysplasia) in the proximal colon is uncertain. Our aims: (1) quantify the relative risk of advanced adenomas in the proximal colon in patients with diminutive adenomas on FS compared to control patients with normal FS; and (2) quantify the risk of finding any adenoma (diminutive or advanced) in the proximal colon in patients with diminutive adenomas on FS compared to control patients with normal FS. METHODS: Inclusion criteria: Consecutive patients with diminutive adenomas on FS and consecutive patients with normal FS were offered screening colonoscopy. Exclusion Criteria: Iron deficiency anemia; occult gastrointestinal bleeding; hematochezia; colonoscopy/barium enema in the past 10 years or normal FS within the past 3 years. Prior to colonoscopy, all patients completed screening CBC and screening FOBT. To insure accurate measurement of polyp diameter, colonoscopic measuring guidewires were used to measure polyp diamter in vivo. DATA ANALYSIS: Relative risk and 95% confidence intervals for the presence of advanced adenoma in the proximal colon and for the presence of any adenomas in the proximal colon were calculated using chi-square analysis with standard software (META-ANAL, McMaster University, Hamiltion, Ontario. RESULTS: 186 patients with normal FS and 190 patients with DAP on FS completed the trial. 5.4% of patients with normal FS (10/186) and 5.8% (11/190) of patients with DAP on FS had advanced adenomas. 22% (40/186) of patients with normal FS and 36% (68/190) of patients with DAP on FS had any size adenoma in the proximal colon. Relative risk for advanced adenoma in the proximal colon of patients with DAP: RR = 1.08(95% CI:0.4 to 2.7);p=0.86). Relative risk for any adenoma in the proximal colon of patients with DAP:RR = 1.7(95%CI:1.2 to 2.4;p=0.003). CONCLUSION: Based on an interim analysis, these data suggest that patients with DAP on FS do not have an increased risk of advanced adenomas in the proximal colon compared to patients with normal FS. However, patients with DAP on FS may have an increased risk of any adenoma in the proximal colon compared to patients with normal FS.