Tumor cell kinetics were studied in vivo in a series of 97 patients with oropharyngeal cancer. The duration of S phase (tS), the labeling index (LI), and the potential doubling time (Tpot) were obtained by flow cytometry measurements of a tumor biopsy obtained after intravenous injection of 200 mg 5-bromodeoxyuridine to the patient. The mean LI was 9.7% (standard deviation [SD], 5.4), the mean tS was 10.1 hours (SD, 3.6), and the mean Tpot was 4.6 days (SD, 3.5). No significant relationship was found between the Tpot or LI and the size of the tumor, nodal status, histological grade, or the site of the primary within the oropharynx. Conversely, aneuploid tumors had longer tS (P < .001), higher LI (P < .001), and shorter Tpot (P < .05) than the diploid tumors. The mean LI and Tpot of the tumors obtained after induction chemotherapy were significantly higher and shorter, respectively, than those measured before any treatment. The data strongly suggest that rapid tumor cell proliferation frequently occurs in oropharyngeal cancer which had responded poorly to chemotherapy.