Pituitary adenomas account for approximately 10 to 15% of primary brain tumors (2,5,30,33,48). The incidence appears higher in African Americans in whom pituitary adenomas account for over 20% of tumors originating in the central nervous system (12,14). Although the incidence varies according to age, sex, and ethnic group, between 0.5 and 8.2 per 100,000 in the population are diagnosed annually with a pituitary adenoma (2,39,42). Autopsy series indicate that pituitary tumors are quite common and that nearly 25% of the population may harbor undiagnosed adenomas (6,46). The majority of these tumors are less than 3 to 5 mm and would not require medical or surgical intervention. More recent series using magnetic resonance imaging (MRI) of healthy subjects indicates that approximately 10% of the population harbors pituitary lesions. Some series report a higher rate of diagnosis among women of childbearing age. However, women may not actually have a higher incidence of pituitary adenoma (2,42). Because disruption of the pituitary axis affects reproductive capacity, women with pituitary adenomas may simply come to clinical attention at a higher rate than men.

Amongst the varying classes of adenomas, prolactinomas and non-functioning adenomas have the highest incidence, and account for nearly two-thirds of all pituitary tumors. Prolactin secreting adenomas account for 40 to 60% of functioning adenomas and are the most common subtype of pituitary tumor diagnosed in adolescents (5). The majority of microadenomas are found in women in their second and third decades. Men generally present later, in their fourth and fifth decades, almost always with macroadenomas.

GH secreting adenomas represent nearly 30% of all functioning tumors. Nearly three quarters of GH secreting adenomas are macroadenomas. Approximately 40 to 60 individuals per million have acromegaly (1,10,41). Between 3 and 4 new cases per million are diagnosed annually (1,4,10,41). Most present in their 3rd to 5th decades after they have been developing symptoms and signs for many years (10). Acromegaly is associated with an increased incidence of cardiovascular, respiratory, cerebrovascular, and malignant disease. Accordingly, studies report an increased risk of mortality compared to the unaffected population (1,4). Although some studies report a higher incidence of several cancers, others have only confirmed an increase risk of colon cancer (16,36,40). There is some evidence that mortality risk may be different between the sexes. Etxabe found a higher mortality rate in men than in women (10). Other reports find similar degrees of increased mortality in both sexes (34). Still others report increased risks of death in men from cardiovascular, respiratory, cerebrovascular, and malignant disease, but only from cerebrovascular disease in women (1).

ACTH adenomas account for 15 to 25% of all functioning adenomas and are the most common pituitary tumors diagnosed in pre-pubertal children (5). The majority of ACTH adenomas, regardless of age, are microadenomas. Approximately 39 individuals per million have Cushing's disease and the annual incidence is estimated at 2.4 per million (11). Cushing's disease is more common in women, most of whom present in their third and fourth decades (11,15). There is a high incidence of hypertension and diabetes mellitus as well as higher vascular disease-related mortality (11,43).