Background & Aims: The flat adenoma may be a more aggressive pathway in colorectal carcinogenesis. Sessile adenomas from the National Polyp Study cohort were reclassified histopathologically as flat or polypoid and compared with initial and surveillance pathology. Methods: A total of 933 sessile adenomas detected during 1980-1990 were reclassified as follows: (1) adenoma thickness (AT): ≤1.3 mm, and (2) adenoma ratio (AR): adenoma thickness <2x normal mucosa thickness. Logistic regression was used to assess whether flat adenomas had an effect on risk for high-grade dysplasia initially, and a Cox proportional hazards model assessed the risk for advanced adenomas at surveillance. Results: The analysis encompassed 8401 person-years of follow-up evaluation. AT and AR measures of adenoma flatness were 95% concordant. By the AT measure, flat adenomas (n = 474) represented 27% of all baseline adenomas. Flat adenomas were found to be no more likely to exhibit high-grade dysplasia than sessile (polypoid) or pedunculated adenomas, the odds ratio for high-grade dysplasia was 1.91 (95% confidence interval [CI], 0.66-5.47; P = 0.23) for sessile (polypoid) vs. flat adenomas and 1.78 (95% CI, 0.63-5.02; P = 0.28) for pedunculated vs. flat adenomas adjusted for size, villous component, and location, and corrected for correlation of risk within an individual patient. Patients with flat adenomas at initial colonoscopy were not at greater risk for advanced adenomas at surveillance compared with those with polypoid adenomas only, the odds ratio was 0.76 (95% CI, 0.4-1.42; P =. 39), adjusted for multiplicity, age, and family history of colorectal cancer. Conclusions: Flat adenomas identified in the National Polyp Study cohort at baseline were not associated with a higher risk for high-grade dysplasia initially, or for advanced adenomas at surveillance.