Background and study aims: Bowel-cleansing studies are frequently underpowered, poorly designed, and use subjective bowel cleansing assessments. Consensus on efficacy, tolerability, and preparation-induced mucosal abnormalities is lacking. This study aimed to clarify the differences in efficacy and preparation-induced mucosal inflammation of sodium phosphate (NaP), colonLYTLEY (PEG), and Picoprep (Pico). Patients and methods: This was a prospective randomized single-blinded trial of ambulatory patients to assess the efficacy of bowel preparation and preparation-induced mucosal inflammation. Proceduralists who were blinded to the preparation taken, assessed both bowel cleansing by using the Ottawa bowel preparation assessment tool and preparation-induced mucosal inflammation. Results: Of the 634 patients, 98 % ingested more than 75 % of the bowel preparation and data were complete for colonic preparation scoring in 99 %. The preparation used, time of procedure, and patient sex all independently impacted on bowel cleansing. NaP was less efficacious than PEG (P < 0.001) and Pico (P < 0.001) for morning procedures whereas all bowel preparations were equally efficacious for afternoon procedures. Preparation-induced mucosal inflammation was 10-fold greater with NaP (P = 0.03) and Pico (P = 0.03) compared with PEG. Conclusions: This is the largest published prospective randomized blinded study on this topic and the first to evaluate the three major classes of preparation with a validated tool. The bowel preparation used, time of procedure, and patient sex all independently impacted on bowel cleansing. NaP gave the worst preparation for morning procedures whereas all preparations were equally effective for afternoon procedures. NaP and Pico induced mucosal inflammation 10-fold more frequently than PEG, a finding that requires further investigation.