Adenomatous polyps of the colon were examined by light and electron microscopy in high risk patients having synchronous carcinoma and in low risk patients having incidental polyps only. There was no difference between these groups when polyp size, degree of dysplasia, and pattern of growth were correlated at light microscopic level. However, the groups differed ultrastructurally; the synchronous polyps showed atypical features, characterized in particular by abnormal secretory droplets and increased numbers of undifferentiated cells. Hyperplastic polyps were also included in the study and showed no atypia. Previously, adenomas from cases of familial polyposis coli have been shown to contain similar abnormal secretory droplets, and it is suggested that this ultrastructural change may be an important marker of early malignancy. Histochemistry and immunohistology may help to characterize the secretory material further.