This study examined the impact of response shift on a psychosocial treatment evaluation of 22 young adult cancer survivors. An age-matched cohort of 54 healthy controls were included in the study to provide a comparison for normative levels and structure of quality-of-life (QOL). It was found that this evaluation of a psychosocial intervention for young adult cancer survivors was notably influenced by response shift phenomenon. Standard analyses suggested that the intervention had no impact on measured aspects of well-being. It did appear to yield an immediate gain in reported global QOL, but seemed to cause a significant decline over time. By considering response shift, it was highlighted that an apparently deleterious effect on QOL was largely a function of response shift. This response shift effect was reflected not only in changes in internal standards, but also in values and in conceptualization of QOL. The intervention seemed to have normalized survivors' conceptualization of QOL so that it was increasingly similar to their age-matched cohort. Future psychosocial intervention research should explicitly consider response shift in a randomized treatment evaluation.