Previous studies are consistent with the hypothesis that aberrant crypt foci (ACF) could be intermediate biomarkers in colorectal carcinogenesis. The present controlled experimental trial was performed to sequentially analyze ACF progression in rat colonic mucosa. F344 rats were administered 2-weekly doses of azoxymethane (15 mg/kg body weight, s.c.) and sacrificed 6, 12, 20, 30 and 36 weeks after the first carcinogen injection. Control groups of untreated rats were sacrificed at the same time points. The number of ACF per area, their multiplicity (number of crypts per focus), ACF frequency and multiplicity according to each colonic site, histology of ACF and macroscopic lesions were recorded. No ACF were found in control animals. In treated animals, the number of ACF per area and the multiplicity progressively and significantly increased throughout the study. ACF were prevalent in the mid colon. Lower frequencies were registered in the distal colon and rectum. ACF were rare in the proximal colon and cecum. By histology, ACF presented superficial and extensive hyperplasia. Tumors were found in the 30th and 36th week. Adenomas and well-differentiated adenocarcinomas were in the distal colon. All proximal neoplasms were signet ring cell carcinomas. In our study, ACF growing features and distribution are not correlated to adenoma and adenocarcinoma distribution. It is conceivable that signet ring cell carcinomas arising in the proximal colon, where ACF are rare, could present a different pathway of growth. The preneoplastic role of ACF and their function as intermediate biomarkers in colorectal carcinogenesis remain to be clarified.