To evaluate the prevalence of lymphocytic infiltrate in a large series of pituitary adenomas, we retrospectively studied tumor tissues from 1400 patients. Based on immunocytochemical data, tumors were classified as PRL (n=411), multihormonal (n=310), immunonegative (n=275), ACTH (n=166), GH (n=137), alpha subunit (n=44), FSH and/or LH, (n=42), and TSH (n=15) adenomas. The lymphocytic infiltrate was diagnosed on histological examination and investigated by immunostaining with anti LCA (human leucocyte common antigen), anti CD45RO (human T cell) and anti CD20 (human B cell) antibodies. Lymphocytic infiltrate was present in 40 adenomas (2.9%), 26 females and 14 males, aged 18 to 77 years (mean+/-SD, 37+/-14 years). The tumors were 19 PRL, 8 multihormonal, 4 GH, 4 alpha subunit, 3 ACTH, and 2 immunonegative adenomas. In PRL adenomas, the sex ratio (female/male) and the age were similar in patients with and without lymphocytic infiltrate (2.8 vs. 4.6 and 29+/-6 years vs. 32+/-11 years, respectively). The frequency of lymphocytic infiltrate was similar in PRL, GH, ACTH and multihormonal adenomas, but lymphocytic infiltrate was significantly more frequent in PRL adenoma than in immunonegative adenoma, and in alpha subunit adenoma than in immunonegative, ACTH and multihormonal adenomas. The lymphocytic cells were almost exclusively T cells. We conclude that lymphocytic infiltrates are rare in pituitary adenomas. Their frequency is not statistically different in major categories of secreting adenomas (PRL, GH, ACTH, multihormonal). Their pathophysiological significance remains to be established.