Predictive assays are presently being developed to identify the patients at highest risk for developing severe late normal-tissue complications. If such patients could be identified prior to treatment, then the doses to those patients could be reduced to lower their complication rate. In addition, patients identified as being relatively radioresistant could receive higher doses without an increase in complications. The aim of the present study was to estimate the magnitude of the dose adjustments that could potentially be made if radiotherapy doses were tailored to the individual patient using a predictive assay of normal-tissue radiosensitivity. The dose adjustments were estimated by re-analyzing data from an earlier study [13] to determine the influence of dose and in vitro fibroblast radiosensitivity on the incidence of severe late normal-tissue complications. Although the dose estimates are preliminary and based on limited data, the results of this study support the concept that a significant therapeutic gain could be achieved for a subset of patients from the use of a predictive assay of normal-tissue radiosensitivity.