Feasibility of Two High-Intensity Interval Training Protocols in Cancer Survivors, Purpose High-intensity interval training (HIIT) is a time-efficient and promising tool for enhancing physical fitness. However, there is lack of research concerning safety and feasibility of HIIT in cancer survivors. Therefore, two different HIIT protocols were investigated in terms of safety, feasibility, and acute exercise responses. Methods Forty cancer survivors (20 breast and 20 prostate cancer survivors, 62.9±9.2 years, BMI 27.4±3.9 kg·m-2, 6-52 weeks after the end of primary therapy) completed a maximal cardiopulmonary exercise test (CPET) and two HIIT protocols on a cycle ergometer: 10x1 min at peak power output (10x1) and 4x4 min at 85–95% peak heart rate (HR) (4x4). Safety (adverse events, AEs), acute physiological responses (HR; blood lactate concentration, bLa) and acute psychological responses (ratings of perceived exertion, RPE; enjoyment) were recorded. Results No major but three minor AEs occurred. Ninety-five percent of participants were able to complete each HIIT protocol. Estimated energy expenditure (159±15 vs. 223±45 kcal, p<0.001), HR (128±20 vs. 139±18 bpm; p<0.001), bLa (5.4±1.0 vs. 5.9±1.9 mmol·l-1; p=0.035) and RPE legs/breathing (13.8±2.0/13.1±2.0 vs. 14.6±2.1/14.3±2.0; p=0.038/0.003) were significantly higher in the 4x4. Enjoyment did not differ between protocols (p=0.301). Conclusions The two HIIT protocols as single sessions appear safe and in the vast majority of breast and prostate cancer survivors after the end of primary therapy also feasible and enjoyable. The 4x4 elicited higher energy expenditure and higher cardio-circulatory and metabolic strain and might therefore be preferred if a high training stimulus is intended. Corresponding author: Prof. Dr. Friederike Rosenberger, Working Group Exercise Oncology, Department of Medical Oncology, National Center for Tumor Diseases (NCT) Heidelberg, Im Neuenheimer Feld 460, 69120 Heidelberg, Germany, Phone: 06221-56-35681, Fax: 06221-56-7109, Email: friederike.rosenberger@nct-heidelberg.de We thank the Dietmar Hopp Foundation for funding the study. Furthermore, we thank the German Cancer Research Center (DKFZ) for the study assessment support. Conflict of Interest disclosure: The authors declare no conflicts of interest. The results of the present study do not constitute endorsement by ACSM. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 24 June 2019 © 2019 American College of Sports Medicine