This randomized controlled, single-blind clinical trial investigates whether virtual reality (VR)-assisted, video game-based exercise training, added to a conventional pulmonary physiotherapy and rehabilitation program, improves functional level, respiratory parameters, and physical fitness in pediatric patients (5-18 years) following hematopoietic stem cell transplantation (HSCT). Participants attended 24 supervised, face-to-face exercise sessions over 8 weeks (3×/week) under physiotherapist supervision. The Experimental Group received game-based exercise using the Nintendo Wii Fit U platform with the Wii Balance Board and the Breathing Labs Breathing Games software (Slovenia) in addition to the conventional program. Outcomes included the Modified Alpha-Fit Pediatric Test Battery (vertical jump, 6-minute walk test, sit-and-reach, flamingo balance), upper-extremity isometric muscle strength (handheld dynamometry) and handgrip strength (Jamar), pulmonary function tests, maximal inspiratory and expiratory mouth pressures, accessory respiratory muscle architecture by myotonometry (MyotonPro: sternocleidomastoid, upper trapezius, pectoralis major), and ultrasound assessment of pleural thickness, diaphragm thickness, and diaphragm excursion.