Hematopoietic stem cell transplantation (HSCT) is an aggressive method of treatment affecting patient's homeostasis. The aim of the study was to evaluate the initial nutritional status of HSCT patients and nutritional status in early posttransplantation period. The prospective study included 100 consecutive patients with hematological malignancies subjected to HSCT. The nutritional status evaluation was made using the nutritional screening scales, anthropometric and biochemical parameters, as well. On the day +7 following HSCT significant decrease in concentration of total protein (5.8 g/dl), albumin (3.6 g/dl) and transferrin (165 mg/dl) were observed (P < 0.001), although the mean body mass/BMI were within the normal range. On the day +14, the biochemical parameters of the nutritional status were even lower (P < 0.001). Poorer nutritional status was associated with worse performance status and mucositis escalation. The adequate nutritional support plan is important element of the whole transplantation procedure.