There is often a discrepancy between quality of life estimates from patients and the general public. These discrepancies are of concern to the disability community, who worry that the public does not understand how valuable life can be for people with disabilities; policy planners, who must decide whose quality of life estimates to use in economic analysis; and practitioners and patients facing difficult medical decisions, who may have to worry that people have difficulty imagining unfamiliar health states. We outline several factors that may contribute to these discrepancies. Discrepancies might occur because patients and the public interpret health state descriptions differently--for example, making different assumptions about the recency of onset of the health state, or about the presence of comorbidities. Discrepancies might also arise if patients adapt to illness and the public does not predict this adaptation; because of response shift in how people use quality of life scales; because of a focusing illusion whereby people forget to consider obvious aspects of unfamiliar health states; because of contrast effects, whereby negative life events make people less bothered by less severe negative life events; and because of different vantage points, with patients viewing their illness in terms of the benefits that would result from regaining health, while the public views the illness in terms of the costs associated with losing good health. Decisions about whose values to measure for the purposes of economic analyses, and how to measure discrepancies, should take these potential contributing factors into account.