Study Type – Therapy (case series) Level of Evidence 4 What’s known on the subject? and What does the study add? A lot of information has been gathered on the subject of complications following urinary bladder augmentation and/or substitution in the recent years. The present study, based on the analysis of 86 patients, gives a critical analysis of these complications (stone formation, bowel obstruction, hematuria‐dysuria syndrome, small bowel bacterial overgrowth, persistent vesico‐ureteral reflux, obstruction at the site of ureteral reimplantation, reservoir perforation, premalignant histological changes, decreased bladder capacity/compliance requiring reaugmentation, etc.). The study adds one more new complication (small bowel colonization following colocystoplasty performed with the cecum and ascending colon) and reports complications in a fairly big (by European standards) cohort of patients with a long follow‐up. OBJECTIVE To evaluate complications after urinary bladder augmentation or substitution in a prospective study in children. PATIENTS AND METHODS Data of 86 patients who underwent urinary bladder augmentation (80 patients) or substitution (6 patients) between 1988 and 2008 at the authors’ institute were analysed. Ileocystoplasty occurred in 32, colocystoplasty in 30 and gastrocystoplasty in 18. Urinary bladder substitution using the large bowel was performed in six patients. All patients empty their bladder by intermittent clean catheterization (ICC), 30 patients via their native urethra and 56 patients through continent abdominal stoma. Mean follow‐up was 8.6 years. Rate of complications and frequency of surgical interventions were statistically analysed (two samples t‐test for proportions) according to the type of gastrointestinal part used. RESULTS In all, 30 patients had no complications. In 56 patients, there were a total of 105 complications (39 bladder stones, 16 stoma complications, 11 bowel obstructions, 5 reservoir perforations, 7 VUR recurrences, 1 ureteral obstruction, 4 vesico‐urethral fistulae, 4 orchido‐epididymitis, 4 haematuria‐dysuria syndrome, 3 decreased bladder capacity/compliance, 3 pre‐malignant histological changes, 1 small bowel bacterial overgrowth and 7 miscellaneous). In 25 patients, more than one complication occurred and required 91 subsequent surgical interventions. Patients with colocystoplasty had significantly more complications (P < 0.05), especially more stone formation rate (P < 0.001) and required more post‐ operative interventions (P < 0.05) than patients with gastrocystoplasty and ileocystoplasty. CONCLUSIONS Urinary bladder augmentation or substitution is associated with a large number of complications, particularly after colocystoplasty. Careful patient selection, adequate preoperative information and life‐long follow‐up are essential for reduction, early detection and management of surgical and metabolic complications in patients with bladder augmentation or substitution.