There are few longitudinal data on nutritional status and body composition of patients undergoing allogeneic hematopoietic cell transplantation (alloHCT). We assessed nutritional status of 105 patients before alloHCT and its course during the early posttransplant period to day þ 30 and day þ 100 via weight history, body mass index (BMI) normalized for gender and age, Subjective Global Assessment, phase angle normalized for gender, age, and BMI, and fat-free and body fat masses. Furthermore, we present a multivariate regression model investigating the impact of factors on body weight. At admission, 23.8% reported significant weight losses (45%) in the previous 6 months, and we noted 31.5% with abnormal age-and sex-adjusted BMI values (p10th, X90th percentiles). BMI decreased significantly (Po0.0001) in both periods by 11% in total, meaning a weight loss of 8.6 ± 5.7 kg. Simultaneously, the patients experienced significant losses (Po0.0001) of both fat-free and body fat masses. Multivariate regression model revealed clinically relevant acute GVHD (parameter estimate 1.43; P ¼ 0.02) and moderate/severe anorexia (parameter estimate 1.07; P ¼ 0.058) as independent factors influencing early weight loss. In conclusion, our results show a significant deterioration in nutritional status during the early post-transplant period. Predominant alloHCT-associated complications such as anorexia and acute GVHD became evident as significant factors influencing nutritional status.