In an endoscopic population screening study for detection of colorectal polyps the diet was registered before ascertaining whether polyps were present. Polyps less than 5 mm in diameter were followed up for 2 years before removal and histologic diagnosis. The relative risk of an increase both in the volume of registered adenomas (excluding new adenomas) and in the total mass of adenomas (including new adenomas) showed a trend towards an inverse relationship with intake of dietary fiber, non-fiber carbohydrate, and cruciferous vegetables, reaching the significance level only for intake of dietary fiber for increase of adenoma volume in men. A trend towards a positive relationship between growth and total fat intake was more inconsistent, although the significance level was reached for the relative risk of increase in adenoma mass for men. These prospective observations with regard to polyp-bearing individuals agree with previous incidence and prevalence data that have indicated a relationship between dietary habits and colorectal carcinogenesis.