BACKGROUND: Cancer-related fatigue (CRF) negatively affects survivors' walking, engagement in physical activity (PA), and quality of life (QoL). Yoga is an effective therapy for treating CRF; however, evidence from large clinical trials regarding how reducing CRF through yoga influences CRF's interference with survivors' walking, engagement in PA, and QoL is not available. We examined the effects of yoga and the mediational influence of CRF on CRF's interference with walking, PA, and QoL among cancer survivors in a multicenter phase III randomized controlled trial. PATIENTS AND METHODS: Cancer survivors (n=410) with insomnia 2 to 24 months posttreatment were randomized to a 4-week yoga intervention-Yoga for Cancer Survivors (YOCAS)-or standard care. A symptom inventory was used to assess how much CRF interfered with survivors' walking, PA, and QoL. The Multidimensional Fatigue Symptom Inventory-Short Form was used to assess CRF. Two-tailed t tests and analyses of covariance were used to examine within-group and between-group differences. Path analysis was used to evaluate mediational relationships between CRF and changes in CRF's interference with walking, PA, and QoL among survivors. RESULTS: Compared with standard care controls, YOCAS participants reported significant improvements in CRF's interference with walking, PA, and QoL at postintervention (all effect size = -0.33; all P≤.05). Improvements in CRF resulting from yoga accounted for significant proportions of the improvements in walking (44%), PA (53%), and QoL (45%; all P≤.05). CONCLUSIONS: A significant proportion (44%-53%) of the YOCAS effect on CRF's interference with walking, PA, and QoL was due to improvements in CRF among cancer survivors. Yoga should be introduced and included as a treatment option for survivors experiencing fatigue. By reducing fatigue, survivors further improve their walking, engagement in PA, and QoL.