A 7-year follow-up study of colonic mucosa proliferation markers was conducted on members of a Native-American family with hereditary nonpolyposis colon cancer. Reproducibility of a tritiated thymidine autoradiography assay for labeling indexes during this 7-year biologically significant time frame was established. A good correlation between tritiated thymidine and a newer technique, bromodeoxyuridine immunoperoxidase staining, was seen. No confounding effect could be attributed to standard colon preparation. On average, both baseline and follow-up values for epithelial proliferation were within the accepted normal range. The presence of essentially normal labeling indexes among colon cancer patients and their high-risk offspring suggests the possibility of the significant effect of dietary factors in reducing proliferation. Such factors may account for the low risk of colon cancer that characterizes the Native-American population in the American Southwest.