Background: The efficient management of the endoscopy unit involves issues of scheduling. The aim of this study was to assess efficient use of an endoscopy unit and its relationship to patient waiting. Methods: Formulas of queueing theory are applied to patient scheduling in endoscopy. The M/M/n queueing model assumes exponential (Markovian) distributions underlying both patient arrival and endoscopy times with n as the number of endoscopists, while the D/M/n model assumes a constant (deterministic) patient arrival rate. Results: As the use of the facility increases, so does the probability that patients will have to wait. It is impossible for an endoscopy facility to have a 100% rate of use without patients being forced to wait. Any increase in the use rate of an endoscopy unit results in a concomitant increase in waiting times. A facility with multiple endoscopists can reduce waiting probability at a higher rate of facility use. A table is included that physicians and administrators can use as a guide to estimate the use rate and waiting characteristics of different types of endoscopy facilities. Conclusions: Endoscopy units must find a balance between patient waiting and underuse of their resources. Endoscopists will occasionally find themselves idle at the benefit of providing patients with tolerable waiting times. It needs to be explained to patients that even under the best of circumstances some waiting cannot be prevented. (Gastrointest Endosc 2000;52:517-24).