The Fontan procedure was first performed in the seventies as a palliation for patients with single ventricle physiology. A feared complication after a Fontan procedure is the development of protein losing enteropathy (PLE). Systemic inflammation has a negative effect on the intestinal barrier integrity, which has supported the use of steroids in this setting. To the best of our knowledge there are no studies linking intestinal inflammation in patients with PLE after Fontan. The objective of this study was to identify the presence of intestinal inflammation measured by FC in patients with PLE after a Fontan procedure. A cross-sectional analysis was performed examining 23 stool samples from 23 Fontan patients for both Fecal alpha-1-antitrypsin (FA1AT) and FC with and without PLE. The median FC was 21 mcg/gm of stool (IQR: 15.7–241 mcg/gm of stool), and the median FA1AT was 40 mg/dL (IQR: 30–220 mg/dL). The median FC and FA1AT were significantly higher in the PLE group than in the Non-PLE group (p = 0.002 and p < 0.0001, respectively). Significantly elevated levels of FC were demonstrated in Fontan patients with PLE, which correlated with the elevated levels of FA1AT. Inversely, levels of FC in Fontan patients without suspected PLE were within the normal range. To our knowledge, this is the first study to demonstrate intestinal inflammation using FC in the setting of PLE within this cohort, and may prove to be useful as a diagnostic tool in its treatment.