To evaluate a 12-week home-based walking intervention among breast cancer survivors and to quantify changes in physical activity (PA) behaviors, body weight, and body composition in response to the intervention. Breast cancer survivors that were in the posttreatment period were randomized to intervention (n=23) or wait-list usual care (n=13). PA was assessed by self-report, and in a study subsample (n=23), by an accelerometer. Intention to treat principles were employed to estimate the intervention effect on PA behaviors, body weight, and body composition. Intervention adherence was calculated as the proportion of exercise sessions completed relative to the number of exercise sessions recommended, as reported each week on walking logs. Thirty-four of 36 women randomized (94%) completed the study. Average intervention adherence over 12 weeks was 94%. Intervention participants reported a significantly greater increase in walking for exercise [+11.9 metabolic equivalent (MET)-h/week] over time than did usual care participants (+1.7 MET-h/week, p=0.01). Objective measures of activity also indicated that intervention participants increased their activity levels over time as compared to usual care participants [i.e., counts/min/day and steps/day (p≤0.04)]. No significant changes in body weight or composition were observed. We found that a 12-week home-based walking intervention was safe and effective for increasing short-term PA levels in breast cancer survivors. Future studies are needed to assess the ability of brief interventions to facilitate the maintenance of increased activity levels and to produce favorable quality of life and risk factor outcomes.