Background/Aims: Mucosal adaptation of the ileum toward colonic epithelium has been reported in pouchitis in ulcerative colitis (UC); however, the clinical characteristics, endoscopic findings, and outcomes in patients with pouchitis with ileal mucosal adaptation are poorly understood. Methods: This was a single-center retrospective study comprising UC patients treated by proctocolectomy with ileal pouch-anal anastomosis who had undergone pouchoscopy at the University of Tsukuba Hospital between 2005 and 2022. Endoscopic phenotypes were evaluated according to the Chicago classification. High-iron diamine staining (HID) was performed to identify sulfomucin (colon-type mucin)-producing goblet cells (GCs) in pouch biopsies. We compared clinical data between patients with (high HID group) and without > 10% sulfomucin-producing GCs in at least one biopsy (low HID group). Results: We reviewed 390 endoscopic examination reports from 50 patients. Focal inflammation was the most common phenotype (78%). Five patients (10%) required diverting ileostomy. Diffuse inflammation and fistula were significant risk factors for diverting ileostomy. The median proportion of sulfomucin-producing GCs on histological analysis of 82 pouch biopsies from 23 patients was 9.9% (range, 0%–93%). The duration of disease was significantly greater in the high HID group compared to the low HID group. The median percentage of sulfomucin-producing GCs was significantly higher in patients with diffuse inflammation or fistula compared to other endoscopic phenotypes (14% vs. 6.0%, P= 0.011). Conclusions Greater proportions of sulfomucin-producing GCs were observed in endoscopic phenotypes associated with poor outcomes in UC, indicating patients with pouchitis showing colonic metaplasia of GCs may benefit from early interventions.