Purpose: Interventions to increase physical activity among breast cancer survivors are needed to improve health and quality of life and possibly to reduce the risk of disease recurrence and early mortality. Therefore, we report the feasibility and preliminary outcomes of a pilot randomized trial designed to increase physical activity in sedentary breast cancer survivors receiving hormone therapy. Methods: Forty-one sedentary women on estrogen receptor modulators or aromatase inhibitors for stage I, II, or IIIA breast cancer were randomly assigned to receive a 12-wk multidisciplinary physical activity behavior change intervention or usual care. Results: Recruitment was 34%, intervention adherence was 99%, and complete follow-up data were obtained on 93%. Most participants (93%) were white with mean age of 53 ± 9 yr. Differences favoring the intervention group were noted for accelerometer physical activity counts (mean difference = 72,103; 95% confidence interval (CI) = 25,383-119,000; effect size (d) = 1.02; P = 0.004), aerobic fitness (mean difference = 2.9; 95% CI = -0.1 to 5.8; d = 0.64; P = 0.058), back/leg muscle strength (mean difference = 12.3; 95% CI = 0.4-15.9; d = 0.81; P = 0.017), waist-to-hip ratio (mean difference = -0.05; 95% CI = -0.01 to -0.08; d = -0.77; P = 0.018), and social well-being (mean difference = 2.0; 95% CI = 0.3-3.8; d = 0.76; P = 0.03). However, the intervention group also reported a greater increase in joint stiffness (mean difference = 1.1; 95% CI = 0.1-2.2; d = 0.70; P = 0.04). Conclusions: A behavior change intervention for breast cancer survivors based on the social cognitive theory is feasible and results in potentially meaningful improvements in physical activity and selected health outcomes. Confirmation in a larger study is warranted. © 2009 The American College of Sports Medicine.