The Internet plays an increasingly important role in health education, providing laypeople with information about health-related topics that range from disease-specific contexts to general health promotion. Compared to traditional health education, the Internet allows the use of multimedia applications that offer promise to enhance individuals' health knowledge and literacy. This study aims at testing the effect of multimedia presentation of health information on learning. Relying on an experimental design, it investigates how retention of information differs for text-only presentation, image-only presentation, and multimedia (text and image) presentation of online health information. Two hundred and forty students were randomly assigned to four groups each exposed to a different website version. Three groups were exposed to the same information using text only, image only, or text and image presentation. A fourth group received unrelated information (control group). Retention was assessed by the means of a recognition test. To examine a possible interaction between website version and recognition test, half of the students received a recognition test in text form and half of them received a recognition test in imagery form. In line with assumptions from Dual Coding Theory, students exposed to the multimedia (text and image) presentation recognized significantly more information than students exposed to the text-only presentation. This did not hold for students exposed to the image-only presentation. The impact of presentation style on retention scores was moderated by the way retention was assessed for image-only presentation, but not for text-only or multimedia presentation. Possible explanations and implications for the design of online health education interventions are discussed.