Although adjuvant breast cancer therapy (including chemotherapy and radiotherapy) increases survival rate, its side-effects can adversely affect patients' health and functional performance. This study, which randomized 55 newly diagnosed early-stage breast cancer patients undergoing adjuvant treatment into 12 weeks of exercise or normal treatment, showed that patients undergoing high-intensity strength training (leg press, four sets of three to five repetitions at ~90% of maximal strength) are not only able to maintain, but can even improve, muscle strength and functional performance. Each session, including warm up, lasted about 20 minutes and was performed twice per week. This training is considered to be safe due to emphasis on a slow, controlled eccentric phase (lowering of the weight). Equally important, patients not participating in strength training experienced large reductions in muscle strength and the ability to perform daily living activities, such as stair climbing and rising from a chair. This study’s results indicate that high-intensity strength training is a simple, time-efficient, low-cost strategy to improve health and functional performance in the face of breast cancer and its treatments.