To evaluate the safety and efficacy of moderate-to-high intensity aerobic training in breast cancer patients receiving neoadjuvant chemotherapy. Twenty patients with stage IIB–IIIC operable breast cancer were randomly assigned to receive doxorubicin plus cyclophosphamide (AC) or AC in combination with aerobic training (AC + AET) (n = 10/group) for 12 weeks. The AC+ AET group performed three supervised aerobic cycle ergometry sessions per week at 60%–100% of exercise capacity (VO_2peak). Safety outcomes included exercise testing as well as treatment- and exercise training-related adverse events (AEs), whereas efficacy outcomes included cardiopulmonary function and patient-reported outcomes (PROs) as measured by a cardiopulmonary exercise test (CPET) and Functional Assessment of Cancer Therapy-Breast (FACT-B) scale. Twelve non-significant ECG abnormalities and three non-life threatening events occurred during CPET procedures. One AE was reported during aerobic training. There were no significant between group differences for clinician-documented events (e.g. pain, nausea) or hematological parameters (p's > 0.05). Attendance and adherence rates to aerobic training were 82% and 66%, respectively. Intention-to-treat analysis indicated that VO_2peak increased by 2.6 ± 3.5 ml/kg/min (+ 13.3%) in the AC + AET group and decreased by 1.5 ± 2.2 ml/kg/min (−8.6%) in the AC group (between group difference, p = 0.001). FACT-B increased 11.1 points in the AC + AET group compared to a 1.5 point decrease in the AC group (between group difference, p = 0.685). Moderate-to-high intensity aerobic training when conducted with one-on-one supervision is a safe adjunct therapy associated with improvements in cardiopulmonary function and select PROs during neoadjuvant chemotherapy.