Aim: Hematopoietic Stem Cell Transplantation (HSCT) is preferred as an aggressive method for the treatment of hematologic cancers. High dose chemotherapy, isolation process and post-transplant complications are important sources of biopsychosocial stress for patients and their relatives. In the literature, the importance of providing psychiatric support and protecting mental health during the HSCT process is emphasized. In this context, this study aims to examine the changes in social support and mental health perceived by patients and their relatives during the HSCT process. Materials and Methods: In the longitudinal and prospective study, the population was determined by considering the number of transplants in the last three years, and the sample size was calculated as a minimum of 13 for each group using 95% confidence and 90% power analysis with the G*Power 3.1 program. The data were collected between June 2022 and 2023 and were conducted in three measurements using the demographic information form, DASS-21, Distress Thermometer (DT) and Multidimensional Perceived Social Support Scale (MSPSS). Results: Participants were 48.77 years old on average and 7.7% of them had a history of psychiatric illness. The mean age of the patients’ relatives was 42.38 years. While the DT scores were not statistically significant, the DASS-21 and the CBAASDS scores were not significant in terms of temporal and group differences (p>0.05). However, one third of the patients and half of the patients’ relatives were found to have significant scores in terms of psychological problems. Conclusions: Addressing physical, psychological and social difficulties in the HSCT process with a multidisciplinary approach is important for early recognition of mental problems.