Between 1978 and 1992 a total of 1689 colorectal adenomas were removed in 1042 patients with no history of previous colorectal neoplasms. One hundred and eighteen patients had at least one adenoma with severe dysplasia. A 'clean colon' was ensured by total colonoscopy in 97% of the patients. A multiple logistic model was used to assess the independent risk factors associated with severe dysplasia. The size of largest adenoma and the extent of villous component were identified as independent patient risk factors associated with high risk of severe dysplasia. The results indicated that the risk of severe dysplasia is high in patients with large and/or villous adenomas and the risk is even higher when the adenomas are found in the sigmoid colon or rectum.