We hypothesized that aberrant crypt foci (ACF) are precursors of colon cancer; their prevention would correlate with cancer risk and their elimination would reduce that risk. In this study we wished to (1) establish the feasibility of stain-enhanced magnification colonoscopy, (2) determine whether colorectal cancer is asociated with increased numbers of ACF, and (3) investigate the natural history of ACF and the durability of their elimination.

Subjects are randomly assigned at the time of stain-enhanced magnification colonoscopy to have any ACF removed or merely observed. Subjects are evenly divided into those with and those without a personal history of colon cancer. ACF are tabulated by an observer blinded to the subject's personal history. All subjects are then re-evaluated after one year by an observer blinded to the original procedure.

Inclusion Criteria:

1. Dept.of Defense Healthcare Beneficiaries
2. Age 18 or over, non-pregnant
3. History of colorectal cancer or other indication for colonoscopic screening
4. At least half of large intestine remaining
5. No use of investigational or chemotherapeutic drugs within 6 mos. -

Exclusion Criteria:

1. History suggesting familial colon cancer syndrome
2. < 6 mos since colon resection or since treatment for colon cancer
3. Anticipated colon surgery within one year of entry
4. Inability to participate in scheduled followup at one year
5. Medical or psychiatric condition which would make patient a poor candidate -