OBJECTIVES A transcription factor Ad4BP/SF‐1 is implicated in the differentiation of gonadotrophs in the pituitary gland, but it is not known whether human pituitary cells express this factor. The present study aimed to disclose (1) whether human pituitary adenomas express Ad4BP/SF‐1, and (2) if this is the case, what kinds of adenoma express the factor.MATERIAL Total RNA was extracted from 23 pituitary adenomas obtained by transsphenoidal surgery, and subjected to Northern blot analyses using cDNAs of bovine Ad4BP/SF‐1, porcine FSH‐β, LH‐β and glycoprotein hormone‐β (GPH‐α) subunits as radiolabelled probes. These adenomas included 13 clinically non‐functioning adenomas, 1 GH/PRL‐producing adenoma, 6 GH‐producing adenomas, 2 PRL‐producing adenomas and 1 ACTH‐producing adenoma.RESULTS The expression of Ad4BP/SF‐1 exactly coincided with the expression of FSH‐β. Thus 5 out of 13 clinically non‐functioning adenomas expressed Ad4BP/SF‐1 and only these 5 adenomas expressed FSH‐β. Interestingly, only one of the GH‐producing adenomas also expressed Ad4BP/SF‐1 as well as FSH‐β. GPH‐α was expressed in 4 non‐functioning adenomas and 2 hormonally functioning adenomas, but did not necessarily coincide with Ad4BP/SF‐1 expression. None of the 23 adenomas we tested expressed LH‐β, probably because LH‐β‐producing adenomas are rather rare.CONCLUSIONS The expression of FSH‐β was parallel with Ad4BP/SF‐1 expression, indicating that the expression of Ad4BP/SF‐1 is restr