Background The relationship between post–heart transplant cachexia and obesity with subsequent morbidity and mortality has not yet been reported. Therefore, the purposes of this study were to: (1) describe change in body mass index (BMI) from before transplant through 5 years after transplant; (2) identify risk factors for increased BMI at 1 year post-transplant; and (3) determine whether post-transplant BMI is associated with post-transplant morbidity and mortality. Methods Patients ( n = 3,540) were from a non-random sample having received a heart transplant between January 1, 1996 and December 31, 2001 at 33 institutions of the Cardiac Transplant Research Database (CTRD). Patients were divided into groups using cut-offs for categories of BMI. Data were assessed according to frequencies, measures of central tendency, Pearson correlations, chi-square tests, multiple regression and stratified actuarial analyses with log-rank tests for comparisons. The level of statistical significance was set at p = 0.05. Results The number of obese patients increased significantly from immediately before heart transplant to 5 years later (17% vs 38%) ( p < 0.0001). Risk factors for increased BMI at 1 year after heart transplant (explaining 56% of variance) included increased BMI at transplant, younger age, black race, non-ischemic etiology of heart disease, Status I at time of transplant and non-use of mycophenolate mofetil. Patients who were underweight or obese at 1 year post-transplant were at greater risk for rejection over time than patients who were of normal weight or overweight ( p = 0.009). Conclusions Both demographic and clinical factors are related to increased BMI at 1 year after heart transplantation. Post-transplant cachexia and obesity are risk factors for poor clinical outcomes after heart transplantation.