BACKGROUND Exercise therapy is being explored in a variety of cancer populations to counteract treatment-related deconditioning. Higher intensity interval protocols are being prescribed to improve physical function and attenuate surrogates of comorbidity in non-cancer populations. The purpose of this study is to explore the safety of higher intensity exercise stimuli on cardiorespiratory fitness (VO2peak) in breast cancer survivors. METHODS Postmenopausal breast cancer survivors were randomized into three groups: supervised aerobic interval training (AIT), supervised continuous moderate exercise training (CMT), and an unsupervised control group (CON). For 6 weeks, AIT exercised between 70 and 100% VO2peak, while CMT exercised between 60 and 70% VO2peak. Both groups followed a matched-work design. RESULTS Thirty-three participants completed the study (age, 57.2 (9) years; weight, 67.6 (12) kg) with no adverse advents. Between-group baseline values were non-significant. VO2peak at baseline (25.3 (5.4) mL·kg(-1)·min(-1)) was below population norms. Compared to CON, cardiorespiratory fitness improved in AIT and CMT by 12% (P < 0.001) with no significant difference between exercise groups. AIT had a greater influence on lower extremity strength (P = 0.026) and body weight (P = 0.031). CONCLUSIONS This pilot study provides evidence that similar to CMT, AIT can safely increase VO2peak in a small group of breast cancer survivors. Further exploration of the benefits of implementing higher intensity training protocols is warranted.