Purpose: To evaluate prospectively the prognostic value of SF2 for local control and survival in patients undergoing radiation therapy for head and neck cancers.Methods and materials: Following informed consent tumor specimens were obtained from 156 patients with primary carcinomas of the head and neck region. The specimens were assessed for the ability to grow in vitro (colony forming efficiency, CFE) and inherent radiosensitivity measured as the surviving fraction at 2 Gy (SF2) using a soft-agar clonogenic assay. Patients were treated mainly with neoadjuvant chemotherapy plus radiation therapy usually as a combination of accelerated external beam and interstitial radiotherapy. The probabilities of local control and survival were analyzed by univariate, bivariate and Cox multivariate analyses.Results: Successful growth was achieved in 110/156 specimens and SF2 values were obtained from 99/156. Eighty four out of these patients underwent radical treatment. The median SF2 value for the 84 tumors was 0.40. At a mean follow-up time of 25 months (range 7-65) the median SF2 value of tumors from 14 patients who developed local recurrence was 0.53, which was significantly higher than the median of 0.38 for tumors from 70 patients without local recurrence (p = 0.015). Tumor SF2 was a significant prognostic factor for local control (p = 0.036), but not for overall survival (p = 0.20). Tumor SF2 was an independent prognostic factor for local control within bivariate and Cox multivariate analyses.Conclusions: This study has shown that tumor radiosensitivity measured as SF2 is a significant prognostic factor for local control in head and neck cancers. Copyright (C) 2000 Elsevier Science Inc.