Recent studies indicate that pre-transplant (Tx) obesity and increased body mass index (BMI) after renal Tx in adults are associated with decreased long-term renal allograft survival. This study examined whether obesity prior to renal Tx or the development of obesity within the 1st year after Tx in children is associated with worse allograft function at 1 year post Tx. This is a retrospective review of medical records from 76 pediatric renal allograft recipients between January 1994 and December 2000. Recipients had their renal allograft functioning at 1 year, and had a glomerular filtration rate (GFR) measured 1 year post Tx. Obesity was defined as body mass index (BMI)≥95th percentile for age, sex, and race. Patients were divided into three groups: (1) children who had BMI≥95th percentile at time of Tx, (2) children who had BMI2) than children in group 2 (57.7±24.5 ml/min per 1.73 m2, P2, PPPP<0.01) than children from other groups. We conclude that pre-Tx obesity is associated with decreased GFR in children at 1 year after Tx. The reason may be persistence or development of post-Tx hypertension in severely obese patients.