Purpose: The Internet presents new opportunities for disseminating decision aids (DAs) with enhanced potential for individualization, but little is known about the feasibility or effectiveness of web-based DAs. Our objective was to measure the impact of an interactive menopause web-based DA on decisional conflict. Methods: We developed an interactive DA with an embedded Markov model (WISDOM: Women's Interactive System about Decision On Menopause) that provides individualized information about menopausal symptoms, disease risks, and the impact of treatments on symptoms and absolute risks. We recruited 232 healthy menopausal women aged 45 to 65 from 4 clinical practices into a randomized, controlled trial comparing WISDOM to a control group (standard care). Those randomized to WISDOM were given unlimited access to the website before their clinic appointment. An individualized summary report (generated by WISDOM) was sent to their physicians. Participants completed baseline and 2-week follow-up questionnaires. The primary outcome was change in decisional conflict, a measure of uncertainty about which course of action to take. Adherence to study protocol was assessed by tracking WISDOM use. Results: 213 participants (92%) completed 2-week assessments. The mean age was 53 years, 75% were college graduates, and 92% were white. Most (92%) reported daily or weekly internet use, 92% had used the internet for over a year, and 70% felt very comfortable using the Internet. After adjusting for age, race, education, and frequency, length, and comfort of Internet use, women randomized to WISDOM had significantly (p=.038) larger reductions in decisional conflict than controls, with a small to moderate effect size (.26). We observed a statistically significant reduction in decisional conflict for the subscales “feeling unclear about values” and “feeling unsupported in decision making”. Additional analyses indicates that participants who were highly adherent to the WISDOM protocol (viewing 80% or more of web pages) had the largest reductions in decisional conflict, however the dose-response effect was not linear. Conclusions: WISDOM reduces decisional conflict about treatment options among menopausal women, especially concerning values clarity and support. Effects were strongest among women who adhered more fully with the treatment protocol.