Helping women make choices to reduce cancer risk and to improve breast health behaviors is important, but the best ways to reach more people with intervention assistance is not known. To test the efficacy of a Web-based intervention designed to help women make better breast health choices, we adapted our previously tested, successful breast health intervention package to be delivered on the Internet, and then we tested it in a randomized trial. We recruited women from the general public to be randomized to either an active intervention group or a delayed intervention control group. The intervention consisted of a specialized Web site providing tailored and personalized risk information to all participants, followed by offers of additional support if needed. Follow-up at 1-year post-randomization revealed significant improvements in mammography screening in intervention women compared with control women (improvement of 13 percentage points). The intervention effects were more powerful in women who increased breast health knowledge and decreased cancer worry during intervention. These data indicate that increases in mammography can be accomplished in population-based mostly insured samples by implementing this simple, low resource intensive intervention.