Circulating human Prolactin (PRL) exists in different variants related to posttranslational modifications, dimerization or association with other serum proteins. Compared to monomeric prolactin these variants usually have little or no biologic activity and include BigBig (BB PRL), Big (B PRL), and Glycosylated forms (G PRL). The aim of the present study was to assess levels of BB PRL, B PRL, little PRL (L PRL) and G PRL in hyperprolactinemic patients with no menstrual alterations or galactorrhea. L PRL, B PRL, and BB PRL were identified by gel filtration chromatography on Sephadex G-100; G PRL and NG PRL were identified by chromatography on Concanavalin A Sepharose. PRL was measured by IRMA DPC. Eleven women, aged 22-50 yrs, were studied for: breast dysplasia (1), controlled hypothyroidism (3), dysmenorrhea (3), microadenoma follow-up (2), and gynecological control (2). Pituitary MRI was normal in all but one patient, who had a microadenoma discovered by Magnetic Resonance Imaging. Six patients had normal L PRL levels, and their hyper PRL was due to excess BPRL or BB PRL. Five patients had increased L PRL levels, but excess G PRL. Patients harboring molecular PRL variants do not present the symptoms typical of the hyperprolactinemic syndrome. Furthermore in patients with clinically controlled prolactinomas the presence of PRL variants should be ruled out to avoid an unnecessary increase of dopamine agonist dosage.