Bowel preparation for colonoscopy requires the administration of large amounts of fluids that can cause fluid and electrolyte disturbances and volume overload, especially in patients with advanced renal failure. Polyethylene glycol-based regimens are considered safe, even in patients with advanced renal failure. However, the incidence of adverse effects (AEs) in routine clinical practice is unknown. The main objective of this study is to carry out a prospective, observational, multicenter clinical registry of renal AE of the preparation for colonoscopy, in patients with advanced renal failure, within the usual clinical practice of the preparation. Variables related to kidney function, bowel-cleansing efficacy, and tolerance will also be recorded. A segmented analysis will be performed in patients with substitutive renal therapy (hemodialysis or peritoneal dialysis).