A new fiberoptic sigmoidoscopic system has been developed that utilizes a disposable sheath to cover and protect all working surfaces of the endoscope from contamination. The reusable part of the endoscope has no air, water, or suction/biopsy channels. These are incorporated in the disposable sheath, which is easily removed after use to provide each patient with a contamination-free endoscope. A prospective, randomized, controlled trial was performed to compare the disposable, sheathed, flexible sigmoidoscope with standard sigmoidoscopes. Clinical evaluations of the new sigmoidoscope system were performed at 15 facilities. Visual analog rating scales were used to record evaluations of endoscope performance and reprocessing by endoscopists and reprocessing personnel. The time to perform procedures, depth of insertion, and total instrument downtime were also recorded. One hundred forty-three procedures (70 standard, 73 sheathed) were performed. No significant difference was found for overall depth of insertion (50 versus 48 cm), although fewer sheathed endoscopes reached to 60 cm than did standard endoscopes (51% versus 30%). The sheathed system had a slightly longer mean procedure time than the standard (5.6 versus 6.7 minutes), but a significantly shortened overall downtime (32.8 vs 8.1 minutes). The standard system was preferred by the endoscopists. Reprocessing personnel preferred the disposable system. The disposable sigmoidoscope system has important advantages of decreased instrument turn-around time and potentially increased staff and patient safety, and future models should be improved to meet physicians' concerns. (Gastrointest Endosc 1995;41:566-72.)