Background: Inadequate bowel preparation can result in missed colonic lesions, cancelled procedures, wasted time, and potential complications. Colonoscopy should be a safe and effective procedure to succeed in diagnosis and treatment of screened lesions. Objective: To recognize predicting factors of colon cleanliness with a patient survey and a cleansing assessment comparing two types of preparations. Material and Methods: A cross-sectional, randomized, multi-centre study was done in 84 asymptomatic patients who underwent to screening colonoscopy. They were divided into two groups according to the bowel cleansing preparation: oral solution of sodium phosphate (NaP) or polyethylene glycol (PEG). All patients answered a semi-quantitative 5-point Likert scale questionnaire regarding their appreciation of the preparation. The cleanliness of the colon was evaluated using the Bitoun's 5-point evaluation score (BS). We used an ordinal regression procedure (ORP) for BS with logit as link function. Results: 84 patients (females 57.1%) with mean age of 50 ± 10 years were included. The BS frequencies were: A=34.5%, B=48.8%, C=14.3% and D= 2.4%. Due to the low frequency we combined C and D groups. The ORP's goodness of fit was p=0.53, the maximum likelihood was reached with 8 interactions, and parallel lines test was p<0.01. The significant predictive factors associated with BS were gender (b male= 1.2, p=0.01), completeness of the treatment (b= 0.095, p<0.01) and type of treatment (b NaP=0.91, p=0.09). Meanwhile, no significant association was found for age (b=-0.001, p=0.98), number of evacuations (b= -0.15, p=0.63) and type of evacuation (b liquid= 0.86, p=0.17 vs semi liquid). The number of polyps found (55.9% of subjects had at least one polyp) was used for assessment of efficacy of bowel preparation, and was analyzed by Poisson regression. The significant factors that predict the polyps number were age (p=0.003), gender (higher in men p=0.003), BS (p=0.02), number of evacuations (inversely related p=0.001) and the use of NaP (p=0.001). The completion of the treatment was not significant. No complications during the procedure were informed due to the preparation. Discussion: Factors associated with successfully cleansed colon were: male gender, use of NaP and completion of the bowel preparation. The efficacy for polyp finding was associated with the cleansing score of the colon, older age, male gender and the use of NaP. Interestingly, the number of evacuations had an inverse correlation with polyp finding.