Obesity is associated with poor health outcomes in the general population, but the evidence surrounding the effect of body mass index (BMI) on postliver transplantation survival is contradictory. The aim of this study was to assess the impact of wait list BMI and BMI changes on the outcomes after liver transplantation. Using the Scientific Registry of Transplant Recipients, we compared survival among different BMI categories and examined the impact of wait list BMI changes on post-transplantation mortality for patients undergoing liver transplantation. Cox proportional hazards multivariate regression was carried out to adjust for confounding factors. Among 38 194 recipients, underweight patients had a poorer survival compared with normal weight (HR = 1.3, 95% CI: 1.13-1.49). Conversely, overweight and mildly obese men experienced better survival rates compared with their lean counterparts (HR = 0.9, 95% CI: 0.84-0.96, and HR = 0.86, 95% CI: 0.79-0.93 respectively). Female patients gaining weight over 18.5 kg/m(2) while on the wait list showed improving outcomes (HR = 0.46, (95% CI: 0.28-0.76)) compared with those remaining underweight. This study supports the harmful impact of underweight on postliver transplant survival, and highlights the need for a specific monitoring and management of candidates with BMIs close to 18.5 kg/m(2) . Obesity does not constitute an absolute contraindication to liver transplantation.