AIMS: While bowel preparation for colonoscopy is the key to successful examination, taking laxatives and showing stools to others causes both physical and mental distress to the patient. Thus, an alternative method to evaluation bowel preparation is necessary. In the current study, we studied the colonic fecal retention by ultrasonography (US) and examined the US finding which reflected completion of BP. MATERIAL AND METHODS: The subjects were outpatients who underwent colonoscopy. This report summarizes the ultrasonographic images of patients who underwent multiple US examinations for all five sites of the colon just before, during, and immediately after bowel preparation. According to the standard protocol, the patients took 2 L of polyethylene glycol-ascorbic acid as a laxative, which was discontinued when the nurse visually judged the stool to be clear. RESULTS: Seven patients in their 50s-80s, none of whom were unable to complete a colonoscopy due to residual feces were included in study. Following bowel preparation, the US images showed anechoic areas with haustration in four or all five areas of the colon. Three of the seven patients received low-dose laxatives (1.1-1.2 L); all three had watery stools in three or more colon areas and none of them were constipated at the time of taking 1 L of laxatives. CONCLUSIONS: Completion of bowel preparation can be assessed by the observation of anechoic areas with haustration in multiple colonic sites by ultrasonography.