The occurrence of hemorrhage from pituitary adenoma (so-called "pituitary apoplexy") was investigated in a consecutive series of 560 cases operated on during the past 30 years. There were 93 cases (16.6%) in which hemorrhage from pituitary adenomas was confirmed either clinically or surgically. These patients were analyzed in terms of age, sex, symptoms and signs, size of tumor, hormonal function, and histological types of adenomas, and computerized tomography findings. In 90 cases (16.6%), hematoma or old bloody fluid was verified within the tumor tissue at surgery. Three other patients presented with subarachnoid hemorrhage, but there was no detectable intratumor hematoma in any of them. Among these 93 patients, 42 (7.5%) showed no evidence of clinical symptoms related to hemorrhage (asymptomatic hemorrhage). Fifty-one patients (9.1%) had definite histories of an acute episode that suggested sudden bleeding (symptomatic hemorrhage: pituitary apoplexy). Thirty-eight patients (6.8%) had a major attack manifested by disturbances of consciousness, hemiparesis, loss of vision, or ocular palsy. In two acromegalic patients, pituitary apoplexy developed during bromocriptine treatment. There was one case of sudden death due to massive hemorrhage from the tumor 14 months after the completion of postoperative radiation therapy. The other 13 symptomatic patients (2.3%) developed a minor attack which included headache, nausea, vomiting, and vertigo. Bleeding from pituitary adenomas was not statistically correlated with any of the following factors: sex, hormonal function of adenomas, and histological types, but it was correlated with age. The number of asymptomatic cases in the third decade was significantly greater than that of the whole group of pituitary adenoma patients in the same decade. The present investigation revealed that the incidence of pituitary apoplexy was unexpectedly high: a major attack in 6.8% of pituitary adenoma patients, a minor attack in 2.3%, and asymptomatic hemorrhage in 7.5% of the cases. This risk of pituitary apoplexy should be kept in mind in treating pituitary adenomas.