The role of body weight change in survival among recipients of hematopoietic stem-cell transplantation is controversial. We assessed the effect of optimizing energy and protein intake on 1-year survival, body weight and body composition, and the effect of body weight and body composition on 1-year survival in 117 patients (57 intervention, 60 control) in a randomized controlled trial. Cox regression was used to study effects of the intervention, weight and body composition on death, relapse, and nonrelapse mortality (NRM). We found no significant effect of intervention versus control on death hazard ratio (HR) 1.05, 95% confidence interval (CI) 0.54-2.04, p = 0.88), relapse (HR 1.15, 95% CI 0.48-2.27, p = 0.75), and NRM (HR 0.95, 95% CI 0.39-2.28, p = 0.90). Body weight, fat-free mass index, body fat mass index and total body water changed over time (p < 0.001), similarly in both groups (0.17 ≤ p ≤ 0.98). In multivariable analyses adjusted for group, gender and age, HRs and 95% CIs per one kilo increase in weight were 1.03 (1.01-1.06) and 1.04 (1.01-1.08) for death and NRM after 1 year (p ≤ 0.02), respectively, and 1.08 (1.01-1.15) for relapse after 3 months (p = 0.02). In conclusion, weight gain is possibly due to fluid retention and is an indicator of a complication in HSCT, rather than a marker of improved nutritional status.