This randomized, controlled pilot trial was carried out to assess the feasibility and efficacy of an aerobic exercise in enhancing physical performance of breast cancer patients after adjuvant treatments. The potential of the training regimen to prevent accompanying bone loss was also assessed. Thirty patients, 41-65 years of age, were randomly assigned into training or control groups shortly after adjuvant chemo- or radiotherapy. The 12-week training included a guided aerobic exercise session once a week (the effective part being either step aerobic- or circuit-training in alternate weeks) and similar home exercise sessions twice a week. Adherence to the guided sessions was 78%, while home training was performed an average 2.1 times per week. Agility assessed with figure-8 running test and peak jumping power showed significant between-group treatment-effects (approximately 5% and approximately 10%, respectively). Judged from the accelerometer data, reaction forces up to six times body weight occurred during the training, which implies that the training could also have potential to affect bone mass. The present exercise regimen turned out to be feasible and effective among breast cancer patients in terms of physical performance. Large controlled trials are necessary to confirm these findings.