The development of computed tomography(CT) and magnetic resonance imaging (MRI) has resulted in the discovery of unsuspected endocrinologically silent pituitary masses(pituitary incidentalomas). The management of these pituitary incidentalomas is controversial. Some lesions may increase in size, compress optic chiasm while others will remain unchanged in size. Five hundred and six patients with pituitary incidentalomas were obtained by questionnaire from March 1999 to May 2000 under the auspices of the Ministry of Health, Labor and Welfare in Japan. In thirty-three patients with pituitary incidentalomas (13.3%) developed tumor enlargement during the mean follow-up period of 45.5 months. Of 115 estimated non-functioning adenomas, 23(20.0%) tumor increased during mean follow up period of 50.7 months (range 10 to 173 months), while 5(5.4%) of 94 estimated Rathke's cysts increased in size during follow up. Pituitary apoplexy was occurred in one patient of 248 patients (0.4%). Pituitary incidentalomas usually follow a benign course. Transsphenoidal adenectomy is indicated for a solid mass attached to optic chiasma. For other patients, MRI every 6 months for the first 2 years, and then yearly may be recommended.