Approximately 60% of Australian women with breast cancer are overweight or obese. A recent meta-analysis found a 33% higher risk for both breast-cancer specific and overall mortality in obese versus non-obese women (p=0.001). It has been strongly suggested that weight loss post-diagnosis may improve breast cancer outcomes; however to date there have been only a small number of trials of weight loss interventions in women with breast cancer. We conducted a feasibility trial to assess the acceptability and efficacy of a telephone-delivered lifestyle-based weight loss program for women following treatment for breast cancer. Ninety women (mean [SD] body mass index: 31.0 [4.3] kg/m²; mean [SD] age: 55.3 [8.7] years; 45.6% post-menopausal at diagnosis; median 16 months post-diagnosis [range: 12-21 months]) diagnosed with stage I-III breast cancer were recruited from the Queensland Cancer Registry (82% of those contacted and eligible). Participants were randomised to the weight loss intervention (up to 16 telephone calls from dietitian and posted intervention materials over 6 months) or usual care. Data were collected at baseline and 6-months (n=74; 82% retention). Mean weight loss (percent of initial body weight) at 6-months was significantly greater in the intervention group (-5.8 [5.0]%) versus the usual care group (-1.0 [5.0]%; p < 0.001). Changes in dietary intake, physical activity, body composition, quality of life, fatigue and body image were also measured. There was high acceptability for the program with 66% of intervention participants rating the program as ‘very helpful’ and 34% rating as ‘helpful’. This is one of only a few trials internationally and the first in Australia to show that weight loss is safe, feasible and acceptable in women following treatment for breast cancer. More research is needed to better understand the impact of weight loss on breast cancer-related outcomes and to understand the mechanisms by which weight loss may improve prognosis.