© 2016 John Wiley & Sons Australia, Ltd Aim: Obesity is associated with poor prognosis and risk of treatment side-effects in breast cancer survivors. This pilot study assessed the feasibility, acceptability, safety and efficacy of a telephone-delivered weight loss intervention, among women (BMI 25–40 kg/m2) following treatment for stage I–III breast cancer, on weight loss (primary outcome), quality of life and treatment-related side-effects (vs usual care). Methods: Ninety women (mean ± SD age: 55.3 ± 8.7years; BMI: 31.0 ± 4.3 kg/m2; 15.9 ± 2.9 months postdiagnosis), recruited from a state-based cancer registry, were randomized to a weight loss (diet and physical activity) intervention (n = 45) or usual care (n = 45). Data collected at baseline and 6 months included weight, body composition, quality of life, fatigue and body image. Acceptability and satisfaction were assessed in intervention participants. Results: Oncologists provided consent to contact 82.6% of patients, with 84.1% of those women contacted and eligible consenting to participate. Compared with usual care, mean weight loss was significantly greater in the intervention arm (−3.1 kg [95% CI, −5.4 to −0.7]; −3.7% baseline weight [95% CI, −6.6 to −0.9]), as were reductions in fat mass (−2.1 kg [95% CI, −4.2 to −0.1]) and waist circumference (−4.0 cm [95% CI, −6.6 to −1.3]). No other statistically significant intervention effects were observed. Participants were highly satisfied with the intervention overall and it is timing in relation to diagnosis/treatment. One reported adverse event (musculoskeletal injury) was attributable to the intervention. Conclusions: This weight loss intervention was feasible, acceptable, safe and effective for women 1–2 years after a breast cancer diagnosis. The effect of weight loss on quality of life and treatment-related side-effects should be examined further in fully-powered studies.