The benefits of bowel preparation prior to capsule endoscopy (CE) are controversial. The aim of this study was to examine whether ingesting a small amount of polyethylene glycol (PEG) during, not before, the CE procedure improves the image quality and the cecal completion rate. A prospective single-blind controlled study was conducted including 59 patients. The initial 32 patients (group A) received no preparation, and the subsequent 27 patients (group B) ingested 500 ml of PEG starting 30 min after swallowing the capsule. The capsule reached the cecum in 65.6% of the patients in group A and 88.9% of the patients in group B (P = 0.038). The use of PEG during CE examination significantly improved the image quality, and this effect was more pronounced in the distal ileum. Ingesting a small amount of PEG during CE examination significantly improves both the CE image quality and the cecal completion rate.