Excess weight and physical inactivity are modifiable risk factors for breast cancer. Behavioral intervention is particularly important among women with an elevated risk profile. This trial tested an intervention that trained women to use a self-monitoring website to increase activity and lose weight. Women with BMI ≥27.5 kg/m2 at elevated breast cancer risk were randomized to the intervention (N = 71) or usual care (N = 34). The intervention group received telephone-based coaching and used web-based self-monitoring tools. At 6 months, significant weight loss was observed in the intervention group (4.7 % loss from starting weight; SD = 4.7 %) relative to usual care (0.4 % gain; SD = 3.0 %) (p < 0.0001). By 12 months, the intervention group had lost 3.7 % of weight (SD = 5.4 %), compared to 1.3 % (SD = 4.2) for usual care (p = 0.003). At 12 months, accelerometer-measured moderate-to-vigorous physical activity increased by 12 min/day (SD = 24) compared to no change in usual care (p = 0.04. In summary, this web- and phone-based approach produced modest but significant improvements in weight and physical activity for women at elevated breast cancer risk.