We aimed to compare the efficacy for bowel cleansing of a split-dose regimen with a low morning dose of PEG solution (Asymmetric; 25% of the dose is given on the day of the procedure and 75% of the dose is given on the day before) with the standard split-dose regimen in patients undergoing screening and surveillance colonoscopy using a low volume bowel preparation (2L PEG-citrate-simethicone plus Bisacodyl). We enrolled consecutive outpatients undergoing screening and surveillance colonoscopy in a randomized, single-blind, non-inferiority clinical trial. Patients were randomly assigned to: group A, asymmetric split dose regimen (1,5 L of PEG + bisacodyl the day before and 0,5 L 4 hours before colonoscopy); group B, symmetric split dose regimen (1 L of PEG + bisacodyl the day before and 1 L 5 hours before colonoscopy). Primary endpoint was the proportion of adequate bowel cleansing. Moreover, all patients filled in a nurse-administered questionnaire assessing compliance, tolerability and safety of bowel preparation.