Patients are increasingly using the internet to access information on health care and services, as well as writing reviews of their own experiences. With patient choice increasing in many health systems, payers, purchasers and providers are attempting to take control of this trend by designing and managing their own patient opinion websites. This essay identifies and explores three aspects of online reviews of health care: the role of patients as judges of health care quality; the motivation behind patients posting reviews; and patients' use of such information. It then discusses how useful patient opinion websites might be in supporting patient choice in health care markets and in expanding voice channels for quality improvement.