To analyze the incidence as well as the natural history of asymptomatic pituitary microadenomas, 450 pituitary glands obtained during routine autopsies were studied. The series consisted of patients of both sexes, without any evidence of pituitary dysfunction, in a broad age range. Thirty-nine tumors were found in 36 pituitary glands, with a peak incidence in the seventh decade of life and an evident predominance in male patients. The medical records and available x-ray films of the skull of these patients with subclinical microadenomas were reviewed for clinical evidence of endocrinopathy and to detect minor radiologic abnormalities of the sella turcica. No significant differences were found in comparing the histologic aspect of these tumors with symptomatic cases. The possibility of these tumors as a preclinical stage of pituitary adenoma or a separate entity is discussed.