Background: For trainees, colonoscopy remains one of the most difficult endoscopic techniques to perform. This is largely due to the huge variability in colonic anatomy, which makes accurate appreciation of instrument looping & tip localisation difficult to judge. Aims: To evaluate the effect of magnetic endoscope imaging (MEI) on performance of colonoscopy by trainee endoscopists. Methods: Consecutive out-patients undergoing colonoscopy were studied. Patients with previous colonic resections were excluded. Two experienced trainees were randomised to perform consecutive examinations either with or without the MEI view. The efficacy of abdominal pressure was accurately assessed using a single sensor coil attached to the endoscopy assistant's hand. The magnetic imager view of each procedure was recorded on computer disk and retrospectively analysed. The clinical findings, total intubation time & extent of intubation, and patient pain score (100mm visual-analogue scale (VAS), 0=none, 100=very severe) were also recorded. Results: 94 patients were studied, 48 with and 46 without the imager view. Total colonoscopy was achieved in all cases except three (blinded to imager view). Frequency of looping was similar in the study groups, although atypical loops were more common in patients examined without the imager view (14/46 vs 1/48; p=0.0001 by Fisher's exact test). The duration of loop formation per patient, however, was significantly less when the trainee was able to see the imager view (median 2.6 mins (0.2-18.7) vs. median 5.0 mins (0.2-27.1); p=0.0056 by Mann-Whitney test) as was the number of attempts taken to straighten the colonoscope per procedure (median 5 (0-19) vs. median 10 (0-57); p=0.001 by Mann-Whitney test). Comparing intubation time in the two study groups, the procedure was also significantly quicker with the benefit of the imager view (median 10.4 mins (4.3-27.4) vs. median 13.3 mins (4-50); p=0.013 by Mann-Whitney test). Abdominal pressure was also more effective when the endoscopist and endoscopy assistant could see the imager view (beneficial 45/62 vs. 19/45; p=0.0026 by Fisher's exact test). Patient pain scores were similar in the two study groups. Conclusions: In trainees, magnetic endoscope imaging significantly improves performance of colonoscopy, making the procedure quicker & allows loops to be accurately assessed and straightened more effectively.