To clarify the relationship between the percentage of c-myc protein-labeled cells, the bromodeoxyuridine (BrdUrd) labeling index (LI) and clinical malignancy in pituitary adenomas, we studied 31 cases of pituitary adenomas. Tumor invasiveness, recurrence, tumor size and the length of illness were evaluated from operative findings, magnetic resonance imaging findings, and the clinical course. Each pituitary adenoma was scored to represent the degree of clinical malignancy. An hour before excision of the tumor, we administered BrdUrd intravenously. Surgical materials were fixed in 70% alcohol and embedded in paraffin. Both hematoxylin and eosin staining and immunohistochemical staining were performed using a monoclonal antibody for both anti-BrdUrd and anti-c-myc protein. Among pituitary adenomas, there was a significantly low percentage of c-myc protein-labeled cells in cases with acromegaly. The percentage of c-myc protein-labeled cells in the pituitary adenomas tended to increase with increase with the total scores of clinical malignancy. The BrdUrd LI was lower than 1% in almost all cases of pituitary adenomas, and it showed no correlation with their clinical malignancy. In conclusion, determination of the percentage of c-myc protein-labeled cells in pituitary adenomas proved to be useful for evaluating their clinical malignancy.