BACKGROUND: Oncologic Internet information quality is considered variable, but no comprehensive analysis exists to support this. We compared the quality of common malignancy Web sites to assess them for language or disease differences and to perform a quality comparison between medical and layperson terminology. METHODS: World Health Organization Health on the Net (HON) principles may be applied to Web sites by using an automated toolbar function. We used the Google search engine ( http://www.google.com ) to assess 10,200 Web sites using the keywords "Breast," "Colorectal," "Stomach," "Liver," "Pancreas," "Bile Duct," "Melanoma," and "Thyroid," plus "cancer," in English, French, German, and Spanish. The searches were then repeated with alternative terms, such as "Bowel" and "Skin cancer." RESULTS: Less than a quarter of Web sites are HON accredited, with significant differences by malignancy type (P < 0.0001), language (P < 0.0001), and tertiles of the first 150 Web sites returned (P < 0.0001). French-language queries resulted in the most accredited Web sites returned. The use of alternative terms resulted in marked differences in accredited Web sites for hepatobiliary cancers. CONCLUSIONS: A lack of validation of most oncologic sites is present, with discrepancies in the quality and number of Web sites across diseases and languages, as well as medical and alternative terms. Physicians should encourage and participate in the development of informative, ethical, and reliable health Web sites on the Internet and direct patients to them.