It is important for surgeons, pathologists, anesthetists and anatomists to know the length of the right main bronchus. It extends from the carina of the trachea to the origin of the right upper lobe bronchus, but an exact method for measuring it has never been described. Using bronchography, the authors measured the length of the right main bronchus in 24 patients. The posteroanterior projection taken at a standard distance (1.8 m) from the patient was used to minimize distortion due to the technique; if present, the distortion would not be more than 5% and would be an increase rather than a decrease in length. The mean length of the right main bronchus was found to be 1.09 cm (range from 0 to 2.9 cm). The clinical importance of this measurement is discussed. The authors conclude that many anatomy textbooks err in describing the length of the right main bronchus as 2.0 to 5.0 cm, but are correct in describing the left main bronchus as being about 5 cm long.