This study aimed to immunohistochemically assess the proliferation activity of pituitary incidentalomas. A series of 52 incidentalomas studied included 22 gonadotroph cell adenomas, 21 null cell adenomas, and 9 clinically silent adenomas (identified as functioning by immunohistochemistry). We also analyzed the differences in proliferation activity between 43 non-functioning pituitary incidentalomas (not including 9 silent adenomas), and 43 symptomatic non-functioning adenomas (NFAs) that caused visual disturbance. Cell proliferation markers were immunostained using monoclonal Ki-67 (MIB-1) antibody and monoclonal anti-topoisomerase II alpha (Topo-II alpha) antibody. The average of MIB-1 labeling indices in pituitary incidentalomas was 0.61% +/- 0.06%. Overall, both MIB-1 and Topo-II alpha labeling indices of the incidentalomas were significantly lower than those of symptomatic NFAs. There were no significant differences in immunopositivity between the two groups based on gender, age, or subtype. The MIB-1 index of the smallest adenoma group in pituitary incidentalomas was significantly lower than in symptomatic NFAs, while the Topo-II alpha incidentaloma was significantly lower than in symptomatic NFAs. Our findings suggest that small or less invasive pituitary incidentalomas should be observed with follow-up MRI. Large or invasive incidentalomas should be surgically treated if the patients show visual disturbances, hypopituitarism, or pituitary apoplexy during the follow-up period.