Abstract Background This prospective longitudinal study examined and compared two measures (prospective and retrospective ones) of post-traumatic growth (PTG) following Hematopoietic Stem-Cell Transplantation (HSCT) and their respective relationships with mental health and psychological disposition. We also tested the hypothesis that unwillingness to be in contact with distressing thoughts and feelings—i.e. experiential avoidance—would moderate the relationship between Post-Traumatic Stress Disorder (PTSD) and growth. Methods This study was carried out with 187 patients. Patients completed the Post-Traumatic Growth Inventory (PTGI) 5 months after HSCT and scales tapping into the five domains of PTGI during hospitalisation and 5 months after HSCT. Mental health and psychological disposition were also assessed prior to hospitalisation. A PTSD scale was administered at the five-month follow-up. Results Prospective and retrospective measures of PTG were weakly correlated. Bayesian pre/post-HSCT comparisons in the prospective measure of PTG revealed substantial to very strong decline in four of the five dimensions assessed. Overall, RCI indicated a reliable increase for 5.6% of patients and a reliable decrease for 40.8% of patients. Confirming that retrospective and prospective measures of PTG reflect different processes, they were not related to the same mental health and psychological disposition variables. Moreover, the hypothesis that acquiring positive outcomes from a potentially traumatic experience, such as HSCT, requires direct confrontation with the source of distress was supported in the case of the retrospective measure of growth but not in the case of the prospective measure growth. Conclusions Retrospective measures such as the PTGI do not appear to assess actual pre- to post-HSCT change. HSCT seems more linked to psychological decline than to growth.