Each day, more than 12.5 million health-related computer searches are conducted on the World Wide Web. Based on a meta-analysis of 24 published surveys, the author estimates that in the developed world, about 39% of persons with cancer are using the Internet, and approximately 2.3 million persons living with cancer worldwide are online. In addition, 15% to 20% of persons with cancer use the Internet "indirectly" through family and friends. Based on a comprehensive review of the literature, the available evidence on how persons with cancer are using the Internet and the effect of Internet use on persons with cancer is summarized. The author distinguishes four areas of Internet use: communication (electronic mail), community (virtual support groups), content (health information on the World Wide Web), and e-commerce. A conceptual framework summarizing the factors involved in a possible link between Internet use and cancer outcomes is presented, and future areas for research are highlighted.