Peak circulatory power (CircP), a product of peak exercise oxygen uptake (VO(2)) and peak mean or systolic arterial blood pressure, has proved to be a strong predictor of poor outcome in adults with congenital heart disease. This study sought to compare CircP with other cardiopulmonary exercise (CPX) test variables and to assess whether CircP is superior in categorizing patients into well-functioning vs. poorly functioning at-risk groups in the pediatric population after a Fontan procedure. The CPX test reports of 50 patients were retrospectively reviewed after the Fontan procedure. The patients were divided into two groups. The well-functioning group included patients in New York Heart Association (NYHA) classes 1 and 2 (n = 36). The poorly functioning at-risk group included patients in NYHA classes 3 and 4 and those with significant indicators or outcomes of a poor prognosis (n = 14). The patients in the well-functioning group had significantly higher CircP values based on mean blood pressure (MBP) (P < 0.001), higher CircP values based on systolic blood pressure (SBP) (P < 0.001), and higher peak VO(2) (P = 0.004) than those in the poorly functioning at-risk group. At a cutoff value less than 2100.4 mmHg/mlO(2)/kg/min, CircP MBP had a sensitivity of 85% in categorizing children to the poorly functioning at-risk group. CircP correlated well with the clinical status of our patients. CircP and peak VO(2) did not differ significantly in ability to identify poorly functioning patients. Further prospective analysis is needed to assess whether CircP can serve as a prognostic marker for the pediatric population after Fontan procedure.