Introduction: There are limited data on exercise capacity and its determinants in pediatric heart transplant patients (HT). Methods: This single-center, retrospective cohort study of patients (<18yo) includes subjects who received their index HT from 1990-2016 and completed a maximal effort cardiopulmonary exercise test (CPET) during adolescence (12-18yo). The primary outcome was maximum oxygen consumption (VO2). Secondary outcomes included anaerobic threshold (AT) and power consumption (PC). Outcomes were evaluated as percent predicted by sex and age. Wilcoxon rank-sum tests were used to test deviation from predicted exercise capacity. Linear regression models were used to assess association of clinical variables with CPET outcomes. Results: Our sample of 86 patients was 53% male (n=46), 68% White (n=53), with a median age at transplant of 10y (IQR [3-14]). The majority had a diagnosis of congenital heart disease (59%, n=51) and 21% (n=18) were supported with VAD prior to HT. The median age at EST was 17y (IQR 14.7, 17.6), mean time from OHT 7.1y (SD 5.3), and most patients were tested using cycle ergometry (87%, n=75). Max VO2 (median=26.3 ml/kg/min, 65% predicted), AT (median=17.5 ml/kg/min, 66% predicted) PC (median 1.8 w/kg, 56% predicted) were significantly lower than 100% predicted (p<0.001 for all, Fig 1). After adjustment for age at transplant and time since transplant, increased BMI (b= -1.32 95% CI [-1.85, -0.79]) and decreased peak heart rate (HR) (b= 0.32 95% CI [0.19, 0.45]) predicted reduced max VO2. Ejection fraction, cardiac index, and pulmonary capillary wedge pressure measured on cardiac catheterization were not associated with max V02. Conclusions: Adolescents after HT demonstrate markedly reduced exercise capacity. While impaired chronotropy has been implicated in impaired exercise capacity in HT, the association of BMI is novel. The identification of this modifiable factor has important clinical implications for future interventions.