Light and electron microscopic findings of six cases of lymphocytic hypophysitis of a collective of 2500 surgical specimens are presented. Five cases were obtained by surgery, one case was obtained from autopsy. Light microscopy revealed an infiltration of mature lymphocytes and plasma cells in the interstitium, partly in the acini, as well as in the posterior lobe and the capsule. The structure of the remaining anterior lobe was normal. The final stage of lymphocytic hypophysitis is fibrosis, which was seen in all cases to varying degrees. The infiltrate consisted mainly of Tiymphocytes, being positive for CK 45 RO and CD 43. Immunocytochemical staining revealed different proportions of residual adenohypophyseal cells. Mainly prolactin reactive cells were observed as were growth hormone reactive cells. By electron microscopy some ruptured acini and damaged adenohypophyseal cells could be seen. Few pituitary cells contained enlarged lysosomal bodies or oncocytic changes. lnflammation causing enlargement of the pituitary leads to patients often being operated under the preoperative diagnosis of a tumor of the sellar region. It is important to identify this special type of hypophysitis, as a different course of treatment is required.