OBJECTIVE Previous studies of Pit-1 expression in human pituitary tumours have produced conflicting results. We have studied expression of Pit-1 mRNA in human pituitary adenomas, as well as in normal human and rat pituitaries, and results were compared with clinical, histological, and immunohistochemical features. In addition, expression of GH, PRL, and TSH-beta mRNA was also studied and compared with Pit-1 gene expression. METHODS The adenomas consisted of 13 GH cell adenomas, 7 PRL cell adenomas, 3 TSH cell adenomas, 4 ACTH cell adenomas, and 10 clinically non-functioning adenomas. Expression of the Pit-1, its isoforms, and each hormone, was studied using the reverse transcription polymerase chain reaction. RESULTS Pit-1 mRNA was expressed not only in normal human and rat pituitaries, but also in all GH, PRL and TSH cell adenomas. There was no correlation between Pit-1 transcripts and biological behaviour or histological findings in these three types of adenoma, suggesting that Pit-1 is generally required for the determination of cell phenotype but is insufficient for the regulation of hormonal activity and tumour growth in these pituitary adenomas. In addition, Pit-1 was also expressed in some ACTH cell (2/4) and non-functioning adenomas (7/10). Although there were no GH, PRL or TSH-beta transcripts in Pit-1 mRNA-negative ACTH cell and non-functioning adenomas, PRL mRNA was detected in all Pit-1 mRNA-positive ACTH cell adenomas and GH, PRL and/or TSH-beta mRNA were found in four of seven Pit-1 mRNA-positive non-functioning adenomas. In contrast, Pit-1 mRNA was expressed without any GH, PRL, or TSH-beta transcripts in only three non-functioning adenomas. These data suggest that expression of Pit-1 mRNA in these two types of adenomas can be mainly attributed to the presence of GH, PRL and/or TSH-beta mRNA expressing cells and that true Pit-1 transcripts found in non-functioning adenomas may be a rare event. Moreover, there were two cases which expressed Pit-1 alpha mRNA, but failed to show other Pit-1 isoform mRNA. There were, however, no clinical or histological differences between these two adenomas showing only Pit-1 alpha mRNA and the others expressing both Pit-1 alpha mRNA and other Pit-1 isoform mRNA. CONCLUSIONS Pit-1 mRNA was expressed not only in GH, PRL and TSH cell adenomas but also in other types of adenoma. However, it is suggested that expression of Pit-1 mRNA in most ACTH cell and non-functioning adenomas can be attributed to GH, PRL and/or TSH-beta mRNA expressing cells. Further studies are necessary to elucidate the role of Pit-1 transcripts in the three non-functioning adenomas without GH, PRL and/or TSH-beta mRNA expression.