There are a lot of studies in which self-report questionnaires are used, showing that cancer patients do not have a lower quality of life than the normal healthy population. This seems to be in contrast with the results of studies in which more extensive interviews have been used and to the everyday experience of physicians, nurses and other caretakers. This phenomenon of underreporting seems to hold true also for other patient groups. Judgment theories explain how the perception of quality of life arises. These theories indicate how the conceptualization of the dimension to be measured, changes under the influence of a (highly significant) life event, such as getting a life threatening disease. These theories hold that there will be a concurrent change in the internalized standard on which the patients base their perception. Thus a real effect, for example a decrease in quality of life as a result of cancer, can be obscured totally. Until an empirically proven solution to this problem has been found, we recommend that answers in questionnaires concerned with quality of life, psychological distress and the like should be approached with due caution.