Background: Aberrant crypt foci (ACF) are the putative precursors to colorectal adenomas and may be useful as biomarkers. Knowledge of their natural history is essential to understanding their potential utility. Objective: Our purpose was to examine ACF detection 1 year after initial observation. Design: We conducted a multicenter study of ACF by using a standardized protocol. ACF in the rectum were assessed and subjects returned 1 year later to evaluate the natural history of the lesions. Setting: Ancillary study to the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Results: Of 78 subjects enrolled, 64 (82%) returned for a repeat examination 1 year later. The mean age was 71 years, 70% were male, and 54% had a history of adenomatous polyps. At the initial examination, 66% of subjects had at least 1 ACF detected in the rectum, with a mean of 2.1 ± 2.3 per person. One year later, 60% of these subjects had at least 1 of the original ACF reidentified, but only 43% of all ACF were reidentified. A total of 56% of subjects had new ACF identified. Limitations: These results are generated from the pilot phase. Improvements or change in technique over time could have influenced the results. Conclusions: A total of 60% of subjects who had ACF continued to have at least one ACF 1 year later, but less than half the specific ACF could be reidentified, and more than 50% of subjects had new ACF. These results imply a considerable dynamic to ACF detection over a 1-year period of observation.