We studied 15 transsphenoidally resected pituitary tissues diagnosed by histologic examination as chronic lymphocytic hypophysitis. Six autopsy-obtained pituitaries of patients who died of nonendocrine diseases also were studied. Tryptase immunohistochemical analysis, which specifically identifies mast cells, demonstrated numerous, randomly distributed multifunctional cells throughout the inflammatory reaction. Several mast cells were located in the vicinity of capillaries; several others were distributed far from the blood vessels. Occasional mast cells also were noted in the nonpathologic anterior and posterior pituitary lobes. Morphometric analysis confirmed that in lymphocytic hypophysitis, the number of mast cells per volume of tissue was significantly increased compared with that of nonpathologic anterior and posterior pituitary lobes. To elucidate the possible role of mast cells in chronic lymphocytic hypophysitis, microvessel densities were assessed quantitatively using immunohistochemical analysis for CD34, a sensitive marker of endothelial cells. The strong positive correlation between numeric density of mast cells and microvessel density per volume of pituitary tissue suggests that mast cell-derived products may influence capillary permeability and angiogenesis, thereby facilitating the access of inflammatory cells to adenohypophysial cells.