This case report demonstrates the application of partial body-weight supported treadmill running in an aggressive rehabilitation program of an ultramarathon runner who had sustained a lumbar disk herniation. Body-weight supported exercise has recognized value during rehabilitation of lower-extremity injuries. In this case we found that the reductions in vertical loading forces achieved through partial body-weight support can also be valuable in the rehabilitation of an injury above the level of support. This motivated runner successfully used weight-supported treadmill training within 1 week of an acute lumbar disk herniation when he was experiencing considerable pain with unsupported walking and lower-extremity weakness. He continued its use until he adequately improved to allow return to his regular overground running program. This case demonstrates how partial body-weight support can allow aggressive running training early after a lumbar disk injury when normal impact forces cannot be tolerated and when leg weakness is a limitation.