METHODS: This phase 2 trial (ClinicalTrials.gov identifier NCT02459132) randomly assigned 63 TCS to usual care (UC) or 12 weeks of supervised HIIT (ie, alternating periods of vigorous-intensity and light-intensity aerobic exercise). The primary outcome was peak aerobic fitness (VO2peak ) assessed via a treadmill-based maximal cardiorespiratory exercise test. Secondary endpoints included CVD risk (eg, Framingham Risk Score), arterial health, parasympathetic nervous system function, and blood-based biomarkers.RESULTS: Postintervention VO2peak data were obtained for 61 participants (97%). HIIT participants attended 99% of the exercise sessions and achieved 98% of the target exercise intensity. Analysis of covariance demonstrated that HIIT was superior to UC for improving VO2peak (adjusted between-group mean difference, 3.7?mL O2 /kg/min; 95% confidence interval, 2.4-5.1 [P<.001]) and multiple secondary outcomes including CVD risk (P?=?.011), arterial thickness (P<.001), arterial stiffness (P<.001), postexercise parasympathetic reactivation (P?=?.001), inflammation (P?=?.045), and low-density lipoprotein (P?=?.014). Overall, HIIT reduced the prevalence of modifiable CVD risk factors by 20% compared with UC.CONCLUSIONS: This randomized trial provides the first evidence that HIIT improves cardiorespiratory fitness, multiple pathways of CVD risk, and surrogate markers of mortality in TCS. These findings have important implications for the management of TCS. Further research concerning the long-term effects of HIIT on CVD morbidity and mortality in TCS is warranted. Cancer 2017;123:4057-65. © 2017 American Cancer Society.BACKGROUND: Testicular cancer survivors (TCS) have an increased risk of treatment-related cardiovascular disease (CVD), which may limit their overall survival. We evaluated the effects of high-intensity aerobic interval training (HIIT) on traditional and novel CVD risk factors and surrogate markers of mortality in a population-based sample of TCS.