BACKGROUND: Androgen deprivation therapy (ADT) and androgen receptor signaling inhibitors (ARSI) are associated with deleterious physical effects, which exercise may mitigate; however, exercise has never been studied in patients initiating treatment with ADT and an ARSI. Our objective was to determine whether supervised exercise prior to and during initial therapy could mitigate adverse effects of ADT plus enzalutamide. METHODS: We conducted a single center trial in patients with recurrent prostate cancer treated with ADT and enzalutamide. We randomized 26 patients to 16 weeks of supervised exercise (aerobic and resistance), starting 4 weeks before initiation of ADT and enzalutamide, or usual care. The primary endpoint was change in peak oxygen uptake (VO2peak) as a measure of cardiorespiratory fitness (CRF). Secondary endpoints were functional capacity, maximal strength, body composition, patient-reported outcomes, safety, and feasibility. Analysis of covariance was used to compare outcomes for groups at Week 17 adjusted for baseline values. RESULTS: The usual care group (N = 13) showed declines from baseline to week 17 in both absolute CRF (-0.31 L/min, -10.9%; p < 0.01) and relative CRF (-3.2 mL/kg/min, -8.9%; p = 0.04); worse fatigue (p = 0.01); and worse quality of life (p = 0.01). At week 17, the exercise group (N = 13) demonstrated improved absolute CRF (between-group change +0.20 L/min, p = 0.05), leg strength (+48.6 kg, p < 0.01) and functional capacity (+21.0 m, p = 0.01) at week 17. CONCLUSIONS: This is the first randomized controlled trial demonstrating a clinically significant decline in CRF in patients initiating ADT and enzalutamide. We show the effectiveness of short-term supervised exercise to mitigate declines in absolute CRF, and improve maximal leg strength and functional capacity. GOV IDENTIFIER: NCT02256111.