During 100 autopsies of patients who had no previous endocrinological problem, selective injections of the pituitary arterial systems were performed. Pituitary glands were examined from 60 male and 40 female patients with an average age of 68 years. Detailed histological and immunocytological studies using peroxidase-antiperoxidase techniques for prolactin (PRL), adrenocorticocotropic hormone (ACTH), follicle-stimulating hormone, thyroid-stimulating hormone, and growth hormone were used. Thirty-three microadenomas were found in 27 patients; 22 were single, 4 were double, and 1 was triple. Fourteen were nonreactive to immunostaining, 10 were reactive to PRL, and 9 were reactive to ACTH. Their diameters ranged between 0.5 and 5.0 mm, and they were located mainly in the lateral wings and the peripheral zone of the anterior pituitary gland. Microvascular study revealed that 22 microadenomas (66%) had direct extraportal arterial supplies. Because the hypothalamic factors are transported to the anterior lobe through the portal vessels, this study suggests that the formation of microadenomas occurs most frequently outside the hypothalamic-hypophyseal portal system and further supports the hypothesis that pituitary adenomas may arise de novo without the influence of hypothalamic factors.