Macroprolactinemia (MP) in which there is a discrepancy between prolactin (P) levels and clinical symptoms has recently generated great interest among physicians of various specialties since not the whole of P is biologically active. The diagnosis of this condition is a topical problem. The purpose of the investigation was to detect high molecular-weight or big big P in patients with hyperprolactinemia (HP) of various genesis and to define its clinical importance. A hundred and one females with HP (P > 700 mU/l) were examined. In each patient, biologically active monomeric P was measured via treatment of polyethylene glycol (PEG) 6000. MP was detected in 23% of the examinees. Among them, only 11 females had clinical symptoms of the disease: 72.7% complained about menstrual irregularities; galactorrhea and headache were observed in 18.3 and 9.0%, respectively. The remaining 12 females had no clinical symptoms of HP. MP in which the clinical symptoms are feebly marked has been found to be not a rare condition. The determination of big big P in HP is of diagnostic value. It is recommended that the serum separation diagnostic technique using PEG should be made routine in the examination of patients with HP, particularly in the discrepancy of biochemical and clinical data and in idiopathic HP.