Cardiopulmonary responses to an 8-week moderate-intensity aerobic exercise intervention and adherence to exercise during and after intervention were assessed in 41 women newly diagnosed with breast cancer undergoing adjuvant therapy. The intervention was primarily aimed at minimizing deconditioning. Women were randomly assigned to the intervention or control group, completed graded exercise tests before and after intervention, and encouraged to continue their exercise postintervention. Over time, only the intervention group showed significant decreases in resting heart rate, resting systolic blood pressure (SBP), P <.05 each, and maximum SBP, P <.02, and an increase in VO2 peak, P <.001, although resting SBP was higher in the intervention group at both timepoints, P <.05. The adherence rate to 8-week exercise intervention was 78.3% with average weekly attendance of 2.4 sessions and 42.7 minutes (27.8 minutes within target heart rate) exercise per session. Overall physical activity levels over 16 weeks postintervention did not differ between 2 groups. However, the within-group analysis indicated that only the intervention group showed a significant increase in voluntary activity, P < .02, and energy expenditure, P < .02, and a decrease in sedentary activity, P < .02. These findings indicate that moderate-intensity aerobic exercise is beneficial in reducing deconditioning of cardiopulmonary responses in newly diagnosed breast cancer women undergoing adjuvant therapy.