Measurements of rectal epithelial proliferation (REP), using tritiated labelled thymidine, correlate with colonic epithelial proliferation, risk for cancer and response to therapies. There have been criticisms regarding its reproducibility and the possible deleterious effects of bowel preparations on this biomarker. We studied paired observations on 7 patients repeated without bowel preparation, 11 repeated after tap-water enema, and 8 repeated after PEG-electrolyte solution or extract of senna purgative and found no significant differences between paired observations. In addition, in a high-risk group for colorectal cancer, 31 persons received PEG or senna preparation and their REP was not significantly different from that of 23 examined without these preparations. Thus, REP is a reproducible biomarker and not affected by several commonly used bowel preparations.