Although the 3H-thymidine labeling index (TLI) has been used extensively as a biologic marker of increased susceptibility to neoplasms or as a prognostic indicator of the clinical outcome in cancer patients, there is still some concern regarding its accuracy and reliability as a marker of cell replication. A study on the prognostic value of TLI in colorectal cancer gave us the ability to evaluate how such a measurement may vary in different laboratories. A total of 150 malignant tumors of the large bowel were studied in the period from 1988 to 1989. Microautoradiography was carried out in tumor fragments taken at surgery. There were only slight differences among the three centers involved in the investigation, mainly as regards the culture medium, exposure time, and the addition of O2 + CO2 during the incubation. No significant difference was observed among TLI values recorded in the three centers (18.3 +/- 0.9; 17.4 +/- 0.9; 15.7 +/- 1.0 mean +/- SE), and the ranges of values were almost identical. Similarly, the numbers of total and of labeled cells counted for each patient were comparable among the three centers. The frequency distribution of TLI showed a peak value between 10 and 19.9% in all three centers. Moreover, more than 80% of the observed values were within the range of 10 to 29.9%. In addition, the neoplastic areas with the highest proliferative activity ("high" TLI) showed a frequency distribution once again rather similar among the three centers.(ABSTRACT TRUNCATED AT 250 WORDS)