The psychometric properties of an instrument, the Activity Scale for Kids-performance (ASKp), were assessed which was proposed to capture physical functioning after allogeneic hematopoietic stem cell transplantation (HSCT). Additionally, this multicenter observational prospective study investigated the influence of clinical correlates focusing on chronic graft-versus-host disease (cGVHD).Patient-reported ASKp, clinician-reported Karnofsky/Lansky status (KPS/PSS), patient characteristics and cGVHD details were assessed of 55 patients with a median age of 12 years at baseline after day +100 post-HSCT and every 3 months during the next 18 months. The psychometric properties were evaluated and ASKp and KPS/PSS status was compared using ANOVAS and multiple regression models.The German version of the ASKp showed good psychometric properties except for ceiling effects. Discrimination ability of the ASKp was good regarding the need for devices but failed to predict cGVHD patients. Both the ASKp and the KPS/PSS were associated with patients after adoptive cell therapy being in need for devices, suffering from overlap cGVHD and from steroid side effects but not with patients' age and gender. In contrast to the KPS/PSS the ASKp only showed significant differences after merging moderate and severe cGHVD patients when comparing them to No-cGVHD (F = 4.050; p = 0.049), being outperformed by the KPS/PSS (F = 20.082; p < 0.001).The ASKp showed no clear advantages compared to KPS/PSS even though economical and patients' effort was higher. Further application range may be limited through ceiling effects. Both should be taken into consideration. Therefore, the results may not support the usage of ASKp after HSCT and rather suggest KPS/PSS, both patient and clinician reported.