Background: The World Wide Web has led to the rapid growth of medical information and continuing medical educational offerings. Ease of access and availability at any time are advantages of the World Wide Web. Existing physician-education sites have often been designed and developed without systematic application of evidence and cognitive-educational theories; little rigorous evaluation has been conducted to determine which design factors are most effective in facilitating improvements in physician performance and patient-health outcomes that might occur as a result of physician participation in Web-based education. Theory and evidence-based Web design principles include the use of: needs assessment, multimodal strategies, interactivity, clinical cases, tailoring, credible evidence-based content, audit and feedback, and patient-education materials. Ease of use and design to support the lowest common technology denominator are also important. Objective: Using these principles, design and develop a Web site including multimodal strategies for improving chlamydial-screening rates among primary care physicians. Methods: We used office-practice data in needs assessment and as an audit/feedback tool. In the intervention introduced in 4 phases over 11 months, we provided a series of interactive, tailored, case vignettes with feedback on peer answers. We included a quality-improvement toolbox including clinical practice guidelines and printable patient education materials. Results:In the formative evaluation of the first 2 chlamydia modules, data regarding the recruitment, enrollment, participation, and reminders have been examined. Preliminary evaluation data from a randomized, controlled trial has tested the effectiveness of this intervention in improving chlamydia screening rates with a significant increase in intervention physicians' chlamydia knowledge, attitude, and skills compared to those of a control group. Conclusions: The application of theory in the development and evaluation of a Web-based continuing medical education intervention offers valuable insight into World Wide Web technology's influence on physician performance and the quality of medical care.