Obesity is a concern in the long-term management of patients following liver transplantation, yet the risk of obesity and the factors that influence its development have not been well defined. We evaluated posttransplantation weight change among a cohort of 774 adults who had their height and weight recorded before liver transplantation at three major centers. Obesity was defined as a body mass index (BMI) of at least 30 kg/m2. Weight at transplantation was adjusted by the amount of ascites removed. Mean BMI increased from 24.8 kg/m2 pretransplantation to 27.0 kg/m2 in the first posttransplantation year, to 28.1 kg/m2 in the second year, and very little with subsequent observation. Among 320 patients who were not obese before transplantation, 21.6% became obese within 2 years after transplantation. On evaluation of numerous potential donor and pretransplantation risk factors, greater recipient BMI, greater donor BMI, and being married were found to be predictors of subsequent obesity (P < .05). Posttransplantation predictors of obesity included absence of acute cellular rejection, higher cumulative prednisone dose in the second year, and cyclosporine-based immunosuppression, although only rejection and prednisone dose remained predictors on multivariate analysis. Despite the marked weight gain after transplantation, prevalence of obesity at 2 years was only slightly greater than in the general US population. Obesity occurred commonly after liver transplantation, sometimes with a striking gain in weight. In addition to BMI at transplantation, donor BMI, marital status, occurrence of acute rejection, and prednisone dose affected the incidence of obesity.