Introduction. Approximately one third of pituitary adenomas are manifested neither by specific symptoms of hormone overproduction nor by elevated blood levels of pituitary hormones. However, these tumours, diagnosed before surgical intervention as clinically non-functioning pituitary adenomas (CNFPAs) express in majority different pituitary hormones, as can be revealed by means of immunohistochemical examination. One of the pituitary hormones which may be expressed in CNFPAs is prolactin (PRL) but the clinical and pathological data on this condition are very scarce. Material and methods. Sixty two pituitary adenomas, diagnosed before surgery as CNFPAs, were immunoassayed with antibodies against PRL, growth hormone (GH), luteinizing hormone (LH), follicle stimulating hormone (FSH), thyrotropin (TSH), alpha subunit (alpha-SU), corticotropin (ACTH) and dopamine receptor type 2. In a proportion of the patients the presurgical concentrations of insulin-like growth factor 1 (IGF-1) were estimated by means of enzyme-amplified chemiluminescence assay. Results. Twenty-three (37.1%) of the examined CNFPAs presented the positive immunoreaction with anti-PRL antibody. Most cases concerned women. Only in two cases (one woman and one man), PRL was the unique hormone expressed in the tumour. In the remaining adenomas PRL immunopositivity was accompanied by GH expression — 17, LH or free bLH — 13, FSH — 2, free a subunit — 4 or by ACTH — 5 tumours. Seven (30.43%) of them were recurrent in comparison with 12.8% PRL-immunonegative recurrent CNFPAs. Dopamine receptors were positively immunostained in all the investigated PRL-immunopositive and all PRL-immunonegative adenomas.

Conclusions. Our data confirm the observations that monohormonal silent prolactinomas are very rare but frequently silent PRL often co-expressed with GH or LH. Although in the whole population of patients with CNFPAs both sexes are equally represented, in the case of silent prolactinomas the female sex is prevalent. The observation of the higher rate of recurrent tumours within PRL-immunopositive adenomas versus PRL-immunonegative CNFPAs has to be confirmed on the larger material.