In order to preoperatively estimate the width of the operating field during transsphenoidal hypophysectomy, we measured the distance between the right and left medial pterygoid plates in 27 normal adults and 11 patients with pituitary adenoma from anteroposterior tomograms. In normal subjects, the mean distance was about 3.2 cm. The distance in patients with acromegaly was not wider than that in normal subjects, but 1 case with giantism showed a much wider distance of 4.1 cm. The measurement of the actual operating field was 2.0 cm in width, about 1.0 cm narrower than the preperative tomogram calculations.