To understand the natural changes in the CT appearance of a pituitary mass after transsphenoidal hypophysectomy of a pituitary tumor, we obtained CT follow-up studies in 12 patients with pituitary adenoma. The heights of the pituitary masses on the coronal sections of each CT study were measured. It was found that the height of the pituitary mass did not return to normal immediately after the operation, despite complete removal of the pituitary tumor. Instead, the height gradually returned to normal between 3 and 4 months after the operation. Results of this study suggest that follow-up CT study of pituitary masses is best performed 3-4 months after transsphenoidal hypophysectomy.