Patients undergoing allo-HCT often experience a substantial loss in physical performance. We have recently published the general effectiveness of an exercise intervention in 105 allo-HCT patients on physical performance and psychosocial well-being. However, predictor variables for differentiated treatment response remained unclear. To determine the impact of basic physical performance on treatment response, we assessed muscle strength and endurance performance at four assessment points before and after allo-HCT. The exercise group started training 2 weeks before admission and ended 6-8 weeks after discharge. Comparing initially fit with unfit classified patients, the fit patients lost 31% of the strength of the knee-extensors, whereas the unfit patients lost only 1% (Po0.05). For endurance capacity, fit patients lost 4% of their walking capability, whereas unfit patients gained 13% (Po0.05). The individual percent change was statistically different at the 0.05 level in all measures of physical performance. Individual training response in allo-HCT patients strongly depends on the initial physical performance level. Unfit patients can be trained safely and may benefit more from this exercise intervention than fit patients. This result is of major clinical relevance and should encourage hematologists to promote exercise even more in impaired/unfit allo-HCT patients.