Successful colonoscopy depends on good preparation of the colon before the procedure. Inadequate preparation may lead to cancelled or repeat procedures and compromise patient safety, quality of care, and cost effectiveness. The primary aim of this study was to isolate factors that affect preparation success, especially in older, more severely ill, bedridden patients. The secondary aim was to examine the possible differences in preparation quality between ambulatory and hospitalized patients and the impact of a staff educational program on the preparation of hospitalized patients for colonoscopy. The study group included 303 consecutive ambulatory patients and 104 hospitalized patients referred for colonoscopy between January and March 2002, before the department introduced an educational program on colonoscopy preparation, and 310 ambulatory patients and 105 hospitalized patients referred for colonoscopy between January and March 2003. All patients completed an ad hoc questionnaire, and the findings were compared between ambulatory and hospitalized patients and between patients treated before and after the educational program. Polyethylene glycol was used significantly more often for colonoscopy preparation in hospitalized patients than in ambulatory patients (53.1% versus 8.8%; P=.0001). The hospitalized group was characterized by more incomplete or repeated colonoscopies and poorer preparation quality. On multivariate analysis, the variables found to be independent predictors of good preparation were successful patient completion of the preparatory procedure according to instructions, colonoscopy performed for follow-up after polypectomy, and preparation with sodium phosphate. The educational intervention had no impact on the quality of preparation. In conclusion, sodium phosphate preparation and complete adherence to preparation instructions are the most important factors for successful colonoscopy preparation. Current preparatory methods for hospitalized and severely ill patients need to be revised.