This paper applies a theory-driven approach to explore why the use of patient-reported outcome (PRO) measures in clinical practice, in particular, health-related quality of life (HRQoL) instruments, has little or no apparent influence on clinical decision making. A theory-driven approach involves combining knowledge of whether and how an intervention works. It is argued that such an approach is currently lacking within the literature evaluating the effectiveness of feeding back HRQoL information to clinicians. The paper identifies a number of mechanisms that might give rise to the expected outcomes that are currently implicit within the design of the intervention and hypotheses specified within the trials evaluating the use of HRQoL measures in clinical practice. It then examines how far current clinical practice matches these mechanisms and in doing so, a number of possible explanations for the lack of impact of HRQoL on clinical decision making are reviewed. The influence of HRQoL information on clinical decision making depends on a large number of factors related to the design of the intervention, patients' and clinicians' desire to discuss HRQoL issues within the consultation and the legitimacy that clinicians give to HRQoL instruments. To date, knowledge of how the feedback of HRQoL information to clinicians might improve doctor-patient communication or clinical decision making has yet to sufficiently inform an assessment of whether these aspects of patient care are improved. The paper concludes by specifying how the feedback of HRQoL information to clinicians might be modified to maximise its impact on clinical decision making.