Human kallikrein 10 (hk10), a secreted serine protease, was reported to function as a tumor suppressor. hK10 immunoexpression has been demonstrated in lactrotrophs and corticotrophs of the nontumorous human adenohypophysis. In the present study, for the first time we report hK10 immunoexpression in various surgically removed corticotroph adenoma subtypes. Specimens were fixed in formalin and embedded in paraffin. Immunostaining was performed using the streptavidin-biotin-peroxidase complex method with an hK10-specific rabbit polyclonal antibody. Results showed that the endocrinologically active adrenocorticotropic hormone (ACTH)-producing pituitary tumors and the silent subtypes were immunopositve for hK10. Intensity of staining varied between the different subtypes. Intensity was lowest in the silent subtypes (silent corticotroph subtypes 1 and 2) compared with nontumorous human adenohypophysial corticotrophs, whereas the endocrinologically active subtypes (ACTH-secreting adenomas, corticotroph carcinomas, Crooke cell adenomas, Crooke cell carcinomas), showed the highest hK10 immunoexpression. Immunopositivity in the nuclei of the ACTH-secreting adenomas and carcinomas, as well as dual cytoplasmic and nuclear localization of hK10 in some of the secreting tumor types was an intriguing finding. Immunoexpression of hK10 in the ACTH-secreting tumors as well as in the Crooke cell tumors was significantly increased when compared with the nonfunctioning tumors and in the corticotrophs of nontumorous pituitaries.