In 129 hyperprolactinemic (PRL 100ng/mL) and 100 normoprolactinemic patients (PRL 0–25 ng/mL), max. PRL (the difference between maximal prolactin (PRL) after thyrotropin releasing hormone (TRH) injection and basal value) was compared with basal PRL and computed tomography (CT) of the sellar region. In 122 hyperprolactinemic patients max. PRL was <100%, while tumor was found in 106 of them. In the remainder seven hyperprolactinemic patients max. PRL was 100% and CT showed no tumor. A significant difference in max. PRL between hyperprolactinemic patients without and those with verified adenoma was found and showed a significant negative correlation with basal PRL. Between 122 hyperprolactinemic patients with max. PRL <100%, mean basal PRL and duration of clinical symptoms were significantly lower in 16 patients with normal CT compared to 106 patients with tumor. All normoprolactinemic patients showed max. PRL 100% and no tumor on CT. PRL stimulation disturbance precedes tumor visualization and represents a decisive diagnostic parameter in hyperprolactinemic patients with no tumor signs.