The World Wide Web has become a significant source of medical information for the public, but there is concern that much of the information is inaccurate, misleading, and unsupported by scientific evidence. This study analyzes the validity of health claims on the World Wide Web for the herbal Opuntia using an evidence-based approach, and supports the observation that individuals must critically assess health information in this relatively new medium of communication. A systematic search by means of nine search engines and online resources of Web sites relating to herbal remedies was conducted and specific sites providing information on the cactus herbal remedy from the genus Opuntia were retrieved. Validity of therapeutic health claims on the Web sites was checked by comparison with reports in the scientific literature subjected to two established quality assessment rating instruments. 184 Web sites from a variety of sources were retrieved and evaluated, and 98 distinct health claims were identified. 53 scientific reports were retrieved to validate claims. 25 involved human subjects, and 28 involved animal or laboratory models. Only 33 (34%) of the claims were addressed in the scientific literature. For 3% of the claims, evidence from the scientific reports was conflicting or contradictory. Of the scientific reports involving human subjects, none met the predefined criteria for high quality as determined by quality assessment rating instruments. Two-thirds of the claims were unsupported by scientific evidence and were based on folklore, or indirect evidence from related sources. Information on herbal remedies such as Opuntia is well represented on the World Wide Web. Health claims on Web sites were numerous and varied widely in subject matter. The determination of the validity of information about claims made for herbals on the Web would help individuals assess their value in medical treatment. However, the Web is conducive to dubious health information and individuals should exercise caution in accepting health claims unsupported by scientific evidence.