Changes of colonic microbiota is involved in colorectal carcinogenesis and could affect composition of patients’ volatome. Mechanical preparation of the bowel strongly decreases the quantity of the gut microbiota and could interfere on the volatile organic compounds (VOCs) composition. This knowledge should be taken into account if the exhaled VOCs composition will be used as a mass screening tool for colorectal cancer. The literature available on this topic in colorectal cancer patients is scarce and the methodology used inconsistent, however there is no evidence that the reduction of the quantity of gut bacteria results in significant changes in the urine and breath VOCs.