Leukemia inhibitory factor (LIF) is a pleiotropic, neuropoietic cytokine found in bovine, murine, and human fetal and adult pituitary cells, mostly in corticotrophs and somatotrophs. In addition, it has been found in a few GH- and ACTH-producing human pituitary adenomas. The aim of our study was to investigate the presence of LIF in various morphologic types of human pituitary adenomas. Ninety-eight operated pituitary adenomas diagnosed by light microscopy and classified by pituitary hormone immunocytochemistry and electron microscopy were studied. Sixty-eight tumors were functioning and included 15 densely granulated (DG) and 10 sparsely granulated (SG) somatotroph (SM) adenomas, 5 lactotroph (LT), 7 mixed SM-LT and 31 corticotroph (CRT) adenomas. The remaining 30 nonfunctioning tumors included 11 gonadotroph (GON) and 19 null cell (NULL) adenomas. For immunocytochemical demonstration of LIF, a specific polyclonal antiserum was applied on formalin-fixed, paraffin-embedded tissues. The immunohistoscore ranging from 1 to 64 grades was determined by multiplying the immunostaining grade (1-4) by the staining intensity grade (1-4), and by the heterogeneity grade (1-4). Ninety adenomas (92%) were variably immunopositive for LIF. LIF was expressed in 77.5% of CRT, 81.8% of GON, 93.3% of DG-SM, and in all SG-SM, LT, SM-LT and NULL adenomas. LT adenomas showed the highest immunostaining grade, followed by SG-SM, GON, NULL, SM-LT, DG-SM, LT and CRT adenomas. GON adenomas showed the highest immunohistoscore, followed by NULL, DG-SM, SM-LT, SG-SM, LT and CRT adenomas. Nonfunctioning tumors showed a significantly higher immunohistoscore compared to functioning adenomas (p < 0.01). We conclude that LIF expression is frequent in all types of functioning and nonfunctioning pituitary adenomas.