In medicine, response shift refers to a change — as a result of an event such as a therapy — in the meaning of one's self-evaluation of quality of life. Due to response shift, estimates of side effects of radiotherapy may be attenuated if patients adapt to treatment toxicities. The purpose of our study was to assess to what extent two components of response shift, scale recalibration and changes in values, occur in early-stage breast cancer patients undergoing radiotherapy and to examine what the implications would be for treatment evaluation. In the week before start of post-operative radiotherapy, 46 patients filled out a questionnaire consisting of quality of life items of the SF-36 and the Rotterdam symptom checklist (RSCL) (pretest). During radiotherapy, patients were asked to fill out the questionnaire twice: a posttest (quality of life at that moment) and a thentest (quality of life before treatment, retrospectively), supposedly using the same internal standard. Changes in values were studied by asking the patients on the two occasions to rate the importance of seven attributes representing various domains of quality of life. Patients were also asked whether their quality of life with respect to the measured aspects had changed since the pretest (subjective transition scores). Significant scale recalibration effects were observed in the areas of fatigue and overall quality of life. When the groups were divided according to their subjective transition scores, significant scale recalibration effects were found in case of worsened quality of life for fatigue and overall quality of life, and in case of improved quality of life for fatigue and psychological well-being. The mean importance ratings remained fairly stable over time, except for ‘skin reactions’, which obtained less importance at the end of radiotherapy than before. In conclusion, effects of scale recalibration were observed that would have significantly affected quality of life evaluations, in that the impact of radiotherapy on fatigue and overall quality of life would have been underestimated. Changes in internal values were observed only for ‘skin reactions’.