In patients with hematological malignancies, exercise is studied as a supportive measure with potential benefits on therapy and disease-related side effects. However, clinical trials have not yet integrated people with Down syndrome (DS), although this disability is associated with an increased risk for hematological malignancies. Therefore, we examined safety and feasibility of a mixed-modality exercise intervention in a male with DS undergoing high-dose chemotherapy for acute lymphoblastic leukemia. Furthermore, physical capacity and fatigue were assessed. Exercise sessions took place 3 times/wk over a 5-week period. Adherence to the exercise program was 100%, and no serious adverse events occurred. In contrast to the training sessions, applied endurance testing was not feasible. Furthermore, maintenance of fatigue level was observed. In conclusion, cancer patients with DS suffering from leukemia should not be excluded from physical activity or exercise programs.