Chronic graft versus host disease (cGVHD) is the underlying cause of late morbidity and mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Conditioning regimen, immunosuppressive therapy, and complications of cGVHD may affect vitamin D status as well as the nutritional and functional status of patients. This is the first prospective study of the impact of vitamin D replacement therapy on the nutritional and functional status of allo-TKMS subjects as well as on the characteristics of cGVHD. A total of 102 allo-HSCT patients (gender: 54 M and 48 F; age 50 (39- 59)) were included in the study, of whom 65 were cGVHD and 37 were control patients. In all participants, nutritional and functional status, vitamin D levels and other laboratory parameters were assessed at the first and control examinations, and depending on the initial serum values, they were prescribed vitamin D replacement therapy.The results of this study showed an incidence of malnutrition aassessed with patient generated subjective global assessment (PG-SGA) in about 40%, impaired functional status assessed with hand grip strength (HGS) and PG-SGA in about 50% and vitamin D deficiency (<50nmol / L) in about 50% and insuficiency (<75nmol / L) in about 30% of all subjects regardless of the presence of cGVHD. A strong correlation between nutritional and functional status was found in the study (P <0.0005). Low HGS (30.2% cGVHD, 10.8% control) and mild loss of muscle mass in out participants indicate an increased risk of developing sarcopenia. Gastrointestinal cGVHD was a risk factor for moderate and severe malnutrition as assessed by PG-SGA (B + C) and malnutrition, cGVHD and cGVHD of the digestive system, liver, and lungs risk factors for low muscle strength. were. We did not find an association of serum vitamin D values with nutritional and functional status or with their change after vitamin D supplementation. The most significant change in serum vitamin D values was present in patients receiving the highest dose of vitamin D supplementation (2000IU). In cGVHD patients, low vitamin D levels were associated with liver cGVHD (P <0.05). Risk factors for vitamin D deficiency in all subjects was myeloablative conditioning, and in cGVHD patients, disease activity. Our results showed that allo-HSCT patients, regardless of the presence of cGVHD, have impaired nutritional and functional status as well as vitamin D deficiency not only in the first months but also in the years after allo-HSCT. Vitamin D supplementation affected the recovery of vitamin D serum values, but not the change in muscle mass and strength.