Progress in the field of diet and colon cancer would be greatly enhanced by the development of a methodology which allowed for the identification and quantification of the early precursor lesions of colon cancer. Recently we described a method (Bird, 1987) consisting of staining the fixed, unsectioned colon with methylene blue for viewing the mucosal surface with the aid of a light microscope. With this methodology we observed early focal lesions in the colons of rodents which had been treated with a colon carcinogen but no lesions in the colons of control rodents. We termed these lesions aberrant crypts (AC). Based on our preliminary observations we hypothesized that AC represent precursor lesions of colon cancer. The findings from our subsequent studies (summarized in this paper) support this contention. We therefore suggest that evaluation of the characteristics of AC will further our understanding of the carcinogenic process as it occurs in the rodent colon and will provide a basis for the investigation of the role of diet in the aetiology of the disease.