BACKGROUND Colorectal cancer is a major health problem worldwide because it is the third most common cancer and the third leading cause of cancer mortality in western countries. Screening for colorectal cancer in asymptomatic patients is crucialfor reducing the incidence and colonoscopy is one of the methods of choice. The ability of colonoscopy in detecting small lesions is clearly influenced by the quality of the colonic preparation. OBJECTIVE To know which are the variablesrelating to the patient and the type ofpreparation that affect the quality of colonic cleansing. METHODS It was designed a cross-sectional study. It was administered a questionnaire for the enrolled subjects to assess the presence offactors that could affect the quality of colonic cleansing. Then they underwent a colonoscopy. The different variables between subjects with adequate or inadequate colonic cleansing were compared. RESULTS We evaluated 277 subjects. In multivariate analysis the only variables that showed significant differences are split dose [OR 0.45 (0.21 to 0.99)] and age [OR 1.02 (1-1.05)]. Obesity showed no significant differences in multivariate analysis [OR 1.84 (0.9-3.78)]. CONCLUSIONS Age and split-dose were the only variables significantly associated with the quality of bowel preparation prior to colonoscopy. Not so constipation or the presence ofdiverticula, so these patients do not require special preparation regimes.