OBJECTIVES: Treatment effects on health-related quality of life (QoL) often differ depending on whether they are measured prospectively (before and after treatment) or retrospectively (after treatment only). It is not clear which of either evaluations is more valid: Prospective evaluations may be biased by scale recalibration (a changed understanding of the response scale), and retrospective evaluations may be biased by recall bias (a wrong assessment of former QoL). METHODS: Based on an analysis of literature, we present an overview on (a) possible biases in prospective and retrospective measurement of QoL, (b) how these biases are named and defined in literature, and (c) current approaches to distinguish scale recalibration and recall bias. RESULTS: The definitions of different biases are inconsistent. Many authors do not clearly distinguish measurement bias from true change. Furthermore, some consider only scale recalibration or only recall bias. There are different approaches for distinguishing scale recalibration and recall bias. We argue that these make too extensive assumptions to be valid. CONCLUSIONS: Much of the current discussion on the validity of prospective and retrospective QoL measurement suffers from unclear definitions, especially of "response shift" and "recall bias", or from neglecting one of the possible biases. We suggest more elaborate definitions for different types of bias, and recommend taking both kinds of bias into consideration when measuring change in QoL. Due to a lack of valid methods, there is not enough evidence on the extent of these biases yet; therefore the best approach for outcomes studies might be to include both prospective and retrospective assessments. In the long run, valid methods need to be developed to determine the most valid method of QoL assessment.