Mirroring the ambiguities noted with other endocrine tumours, histology is a poor predictor of the invasiveness of pituitary adenomas. Accordingly, the proteins p27Kip1 (p27) and Ki-67, which respectively block cell-cycle progression and reflect proliferative activity of cells, were studied immunohistochemically in 123 specimens of pituitary adenomas including 57 clinically verified invasive cases. Only Ki-67 had a significant association with invasiveness; only p27 differed significantly between functioning and nonfunctioning adenomas, with a higher frequency of cell labelling in the former. No relationship between Ki-67 and p27 was obtained. Ki-67 detected by MIB-1 is thus an important marker related to the invasive potential of adenomas, and thereby may be helpful in planning postoperative management of patients with pituitary adenomas.