Body weight is the traditional metric for matching donor and recipient size for pediatric heart transplantation (pHT). We hypothesized that mismatch in body mass index (BMI) or body surface area (BSA) rather than weight is better associated with outcomes of transplantation and therefore should be used for donor-recipient size matching. Analysis of the United Network for Organ Sharing database limited to pHT recipients was performed. Donor and recipient mismatch groups were created for weight, BMI, and BSA ratios. Differences in recipient characteristics between each cohort and the impact of mismatch on outcomes were statistically analyzed. A total of 4,465 patients were included in the analysis of which 43% had congenital heart disease (CHD). There were significant differences in patient characteristics by matching, independent of the matching parameter. Multivariable regression analysis showed that a low donor-recipient BMI ratio (compared to normal) (CHD OR 1.70; non-CHD 2.78) was a predictor of one-year mortality (all P < .001) in both CHD and non-CHD cohorts. Low BMI ratio was also associated with worse long-term survival in non-CHD groups, but not in the CHD cohort. Weight and BSA ratio did not predict one year or long-term survival. The use of low BMI donors compared to recipient may predict poor early and long-term survival and therefore should be avoided in pHT. The use of BMI matching may improve donor-recipient matching in pHT.