Thirty patients treated for squamous cell carcinoma of the upper aerodigestive tract were reviewed for a cell kinetics study using bromodeoxyuridine (BrdU). Fresh specimens were obtained from the primary site of the tumors and incubated in Dulbecco's modified Eagle's medium containing BrdU to label S-phase tumor cells. The tumor specimens were stained immunohistochemically with monoclonal antibody against BrdU. The results demonstrated that the BrdU labeling index (BrdU LI) in these cases (470.3 +/- 118.2) was greater than the BrdU LI in normal mucosa (162 +/- 52.4). In a comparison of the clinical data and histopathologic features, the BrdU LI showed an inverse correlation with patient age (r = -.558; p < .01) and a direct correlation with the frequency of mitoses (r = .411; p < .05). Although the significance of the BrdU LI as a prognostic factor cannot be determined at present, this technique provides a simple in vitro test that avoids the potential hazards of in vivo labeling or the use of radioactive materials.