Rectal mucosal (epithelial) proliferation labeling indices (LI) have been widely used as a risk marker for colorectal cancer and as an intermediate end-point in chemoprevention studies. The purpose of this study was to determine whether the presence of cancer preoperatively altered the LI and whether these LI could be used over the long term as a marker for colon cancer. We studied 25 patients (18 with colorectal cancer and 7 with benign colonic diseases) who were admitted for colonic resection. Biopsies for thymidine LI were taken before, during, and 4 and 7 months after the operation. The preoperative LI of cancer patients was higher, but not significantly, than that of noncancer patients (5.22 +/- 3.54 and 4.11 +/- 1.34%, respectively, p = 0.28). The intraoperative LI was significantly higher than the preoperative LI (8.08 +/- 4.00 and 4.90 +/- 3.07%, respectively; p = 0.004). After 4 and 7 months, the LI was not significantly different from the preoperative LI (p = 0.60 and 0.89, respectively). Resection of a colonic segment did not affect the level of proliferation over time. Therefore, it is unlikely that LI can be used as a marker to predict local recurrence after curative resection of colorectal cancer.