The purpose of this study is to clarify the nature of macroprolactin which has recently been highlighted because of the high prevalence and a diagnostic pitfall of hyperprolactinemia. The results of this study are beneficial for the patients to whom unnecessary medications and unnecessary radiological examinations have been performed. We used the remained serum samples taken for routine prolactin determination during the past several years. Since the blood was basically taken to examine the causes of hyperprolactinemia and only remained serum samples were used for this study, the Ethics Committee of Kobe City General Hospital judged that it was not required to obtain approval number.