This report examines the 3H-thymidine-labeling index (LI), the flow cytometric S-phase cell fraction, and DNA ploidy in the normal colorectal mucosa of three groups of subjects: normal subjects (group A), patients with adenomas (group B) and patients with cancer (group C). The total LI and the LI for crypt compartment were investigated. The former LI was similar in the three groups while that for crypt compartment was significantly higher in the patients with adenomas (compartment 3), and in those with cancer (compartments 3 and 4). All of the mucosas were diploid and no significant relationship was found between the LI and the flow cytometric S-phase values, considered for each group. These data suggest that an upwards expansion of the proliferative compartment seems to be an indicator of cancer risk. In contrast, the variations of the ploidy and S-phase fraction do not appear to be important risk factors in the three groups of subjects studied.