Little data is available on the effect of mindfulness amongst pediatric hematology-oncology professionals. The purpose was to further document change in biological and psychological stress following a mindfulness-based stress reduction (MBSR) program. We led two pre-post interventional studies (n = 12 and n = 25) and measured changes on hair cortisol concentrations, perceived stress, psychological distress and burnout. Professionals did not change on biological stress (d = 0.04), but improved on self-reported measures (median d = 0.58). Effects were maintained over 3 months for psychological distress, anxiety, depression, and burnout (median d = 0.66). Effects were larger if trainees participated to the retreat and if they reported higher baseline perceived stress. In pediatric hematology-oncology professionals, an MBSR program was related with improvements in self-reported stress over 3 months. Components of the program and characteristics of trainees may influence the impact of MBSR. •We evaluated changes following a Mindfulness-Based Stress Reduction (MBSR) program in hematology-oncology professionals.•We observed improvements in psychological distress, anxiety, depression, and burnout, but no change in hair cortisol.•In this understudied population, MBSR may modify primarily the experience of stress.•Baseline stress and participation to a silent retreat may increase the effect of MBSR.