Purpose: To describe the histopathological findings in the megarectosigmoid of pediatric patients with severe chronic idiopathic constipation (CIC) who underwent a transanal proximal rectosigmoid resection. Methods: This study included pediatric patients diagnosed with severe CIC with megarectosigmoid, who underwent transanal proximal rectosigmoidectomy at the Colorectal Center for Children at UPMC Children’s Hospital of Pittsburgh between July 2017 and May 2018. A retrospective case review was conducted. Histopathological analysis of the resected specimens was performed circumferentially at both the proximal and distal margins. Longitudinal sections throughout the entire specimen length were also assessed. Muscle thickness and nerve fiber size were compared with age-matched controls. Statistical analyses were carried out using SPSS version 21.0. Results: A total of twelve patients met the inclusion criteria. The median age at the time of surgery was 8.6 years, with a predominance of male patients. The median duration of constipation was 5.5 years. Histopathological examination revealed significant hypertrophy of both the inner and outer muscular layers of the muscularis propria, as well as thickened nerve fibers and an increased width of the lamina propria when compared to controls. The increased thickness of the lamina propria was primarily attributed to fibrosis. A significant difference (p<0.05) in the thickness of the outer muscularis propria, nerve fiber size, and lamina propria was observed when comparing the distal (rectum) with the proximal margin (sigmoid) of the specimen. Fibrosis was universally present in all cases, predominantly within the lamina propria, and was more pronounced in the distal than the proximal segment. No morphometric analysis was performed for this assessment. Conclusion: Histopathological findings in pediatric patients with severe CIC and megarectosigmoid demonstrated marked muscular hypertrophy, thickened nerve fibers, and fibrosis within the lamina propria, with the most pronounced alterations observed in the resected rectal segment. These findings suggest a more advanced disease process within the rectum. This report is the first to document fibrosis in this patient population, raising concerns regarding potential irreversible changes that may only be amenable to surgical intervention.