Background: The importance of patient perspective in disease and healthcare is now widely recognized. It is common when studying therapeutic interventions to employ patient assessments of health. Patient-reported outcome is an umbrella term applicable to any health data reported by the patient, such as symptoms, functional status, satisfaction with therapy, or treatment adherence (Acquadro et al 2003). PRO measures extend the range of patient outcomes that can be assessed beyond traditional measure of survival or objective clinical efficacy to capture the patient perspective on symptoms overall health status and the impact of disease and treatment on quality of life (Acquadro et al 2003; Walters 2009). Given their increasing use in appraising intervention effectiveness, understanding the potential bias and the magnitude of the bias they may be vulnerable to is important in careful synthesis and interpretation of evidence. Objective: To explore the potential bias in patient reported outcomes in the treatment of chronic conditionswhere patients are not blind to treatment. Methods: Analysis of data from trials included in 17 meta-analyses examining interventions to treat chronic conditions. For each study in the included meta-analyses outcomes were subdivided into objectively assessed, or a PRO.Meta-analyses were included where objective measures showed no statistically significant effect of the intervention but also included a PRO. Studies were also defined as having been blinded or not blinded. Logistic regression models were used to compare intervention effects on PROs in trials with and without blinding. Results: Lack of blinding was associated with considerably greater estimates of effect in PROs for patients suffering from chronic conditions. Conclusions: Particular caution needs to be exercised in the interpretation of patient reported outcomes where there is a lack of blinding this is particularly so in reviews where there is also a lack of studies reporting outcomes for objective measures for treatment effectiveness.