Physical inactivity and being overweight or obese are lifestyle factors that put breast cancer survivors at a higher risk for a cancer recurrence and/or development of other chronic diseases. Despite this, there is limited research that has identified effective lifestyle interventions aimed specifically at weight loss in breast cancer survivors. This pilot study is a single-arm experimental pre–post test design, conducted from November 2009 to July 2010, that tested the efficacy of a 24-week group-based lifestyle intervention modeled on the Diabetes Prevention Program in early stage breast cancer survivors (N=14). The intervention included 16 diet sessions led by a registered dietitian and 150 min/wk of moderate-to-vigorous exercise. Study outcome measures were completed at baseline, 24, and 36 weeks (nonintervention follow-up). The primary outcome was change in body weight, and secondary outcomes were change in body composition, aerobic fitness, dietary intake, and blood biomarkers. Overall, participants were postmenopausal women aged 54.6±8.3 years with obesity (body mass index 30.1±3.6), and had completed adjuvant cancer treatment 2 years prior. Results showed an average weight loss of 3.8±5.0 kg and a decrease in body mass index, percent body fat, and waist and hip circumferences at 24 weeks and an additional mean weight loss of 0.8±1.2 kg at 36 weeks. In exploratory analysis, participants who lost >7% body weight were older and attended a greater percentage of diet and supervised exercise sessions. There were no significant changes in any of the blood biomarkers at 24 and 36 weeks; however, the results provide a measure of expected effect size for future research studies. This pilot study demonstrated the efficacy of a lifestyle intervention based on the Diabetes Prevention Program in early stage breast cancer survivors and represents an innovative clinical intervention for dietetics practitioners to address the unmet need for programs.