The rapid growth of the Internet has triggered an information revolution of unprecedented magnitude. Despite its obvious benefits, the increase in the availability of information could also result in many potentially harmful effects on both consumers and health professionals who do not use it appropriately. To identify instruments used to rate Web sites providing health information on the Internet, rate criteria used by them, establish the degree of validation of the instruments, and provide future directions for research in this area. MEDLINE (1966-1997), CINAHL (1982-1997), HEALTH (1975-1997), Information Science Abstracts (1966 to September 1995), Library and Information Science Abstracts (1969-1995), and Library Literature (1984-1996); the search engines Lycos, Excite, Open Text, Yahoo, HotBot, Infoseek, and Magellan; Internet discussion lists; meeting proceedings; multiple Web pages; and reference lists. INSTRUMENT SELECTION: Instruments used at least once to rate the quality of Web sites providing health information with their rating criteria available on the Internet. The name of the developing organization, Internet address, rating criteria, information on the development of the instrument, number and background of people generating the assessments, and data on the validity and reliability of the measurements. A total of 47 rating instruments were identified. Fourteen provided a description of the criteria used to produce the ratings, and 5 of these provided instructions for their use. None of the instruments identified provided information on the interobserver reliability and construct validity of the measurements. Many incompletely developed instruments to evaluate health information exist on the Internet. It is unclear, however, whether they should exist in the first place, whether they measure what they claim to measure, or whether they lead to more good than harm.