We describe the effects of an aerobic exercise program designed to improve the physical performance of patients undergoing bone marrow transplantation. Twenty patients entered the rehabilitation program, consisting of walking on a treadmill, and carried it out for 6 weeks. Patients started the training program 30 +/- 6 days (range 18-42) post-BMT. By the end of the program we observed a significant improvement in maximal physical performance and maximum walking distance, and a significant lowering of the heart rate with equivalent workloads (P for all significances < 0.001). All participants of the program reached a peak performance (calculated in metabolic equivalents, METs) more than sufficient for carrying out all basic activities of daily living. These results contrast with literature reports indicating that spontaneous recovery of physical functioning after BMT can take many months and that about 30% of patients experience long-lasting impairment of physical performance. We conclude that that fatigue and loss of physical performance in patients undergoing BMT can be corrected with adequate rehabilitative measures.