Most comparative studies on different preparations for colonic cleanliness use unvalidated scales with terms as "excellent," "good," "fair," and "poor" which lack standardized definitions. The Boston Bowel Preparation Scale (BBPS) is a valid and simple measure of bowel preparation. To compare three different bowel preparations [polyethylenglycol (PEG), sodium phosphates, and PEG + bisacodyl tablets] using BBPS. Patients undergoing screening colonoscopies were included during a period of 6 months. Every patient was scored according to BBPS. Six endoscopists prospectively enrolled 374 patients (200 female and 174 male, median age 56.9 years old). Physicians chose the preparation method, and in spite of not being a randomized trial, numbers in each group allowed statistical analysis: PEG 116patients (31%), sodium phosphates 212 (56.7%) and bisacodyl 46 (12.3%). There was statistical difference between the three preparations in favor of the 4 litre solution of PEG, with a median score of 7, towards sodium phosphates and bisacodyl, with median scores of 6 (P < 0.001). Depending on bowel preparation, there were positive polyp findings in 40% of colonoscopies with PEG, 26% with sodium phosphates and 22% with bisacodyl (P = 0.01). Afternoon procedures had better preparation scores than morning procedures, 7 and 6, respectively (P < 0.001). In the afternoon colonoscopies, there were no statisticaIly significant differences between the preparations (P = 0.12) or polyp findings (P = 0.13). In the morning shift, PEG prepared patients had better scores (score of 6) when compared to sodium phosphate (score of 5) and bisacodyl (score of 6) (P = 0.001). We also noticed that the shorter the time interval between the last intake of the preparation and the procedure, the better the score. Bowel preparations for colonoscopy with PEG are significantly better than sodium phosphate and bisacodyl preparations, with higher BBPS scores and polyp detection rates.