Improving patient education focusing on bowel preparation before a colonoscopy leads to cleaner colons. Endoscopy units must obtain informed consent and perform a risk assessment for sedative use prior to a colonoscopy. The current practice in the Netherlands to achieve these goals is nurse counseling in an outpatient setting. This is costly and has disadvantages in terms of uniformity and time consumption for both the patient and the hospital. The hypothesis is that computer-based education with use of video and 3D animations may replace nurse counseling in most cases, without losing quality of bowel cleanliness during colonoscopy. This multicenter, randomized, endoscopist blinded clinical trial evaluates a primary outcome measure (bowel preparation) during colonoscopy. Secondary outcome measures are sickness absence, patient anxiety after instruction and prior to colonoscopy, patient satisfaction and information re-call. The study will be performed in four endoscopy units of different levels (rural, urban, and tertiary). Inclusion criteria are adult age and referral for complete colonoscopy. Exclusion criteria are Dutch illiteracy, audiovisual handicaps or mental disabilities and no (peers with) internet access. This trial aims to establish online computer-based education as tool for patient education prior to a colonoscopy. By choosing a direct comparison with the standard of care (nurse counseling), both endoscopic quality measures and patient related outcome measures can be evaluated.