Abstract. The clinical influence of macroprolactin (MPRL) is not clearly understood and the rate of patients potentially affected by MPRL is unknown. We investigated the influence of MPRL on the onset of galactorrhea and estimated the rate of patients with a proportion of MPRL fraction that may possibly affect galactorrhea. Data of patients with obstetric or gynecological symptoms who had undergone PRL fractionation testing were retrospectively analyzed. To evaluate factors influencing galactorrhea, a multivariate logistic regression analysis was performed and the adjusted odds ratios of MPRL for galactorrhea were calculated. Cutoff values for the total PRL level and the proportion of MPRL fractions for galactorrhea were determined by ROC analysis using a multivariate logistic model. The prevalence of patients with a proportion of MPRL fraction greater than or equal to the cutoff value for galactorrhea was estimated. The median proportion of MPRL fraction was 30.1% and increased as PRL level increased. Total PRL and MPRL had a significant influence on the onset of galactorrhea and the adjusted odds ratio was 1.09 in total PRL and 0.94 in MPRL. The rate of patients with a proportion of MPRL fraction that may possibly affect galactorrhea was estimated to be 33.5% of the study population, and thus found to be twelve times or more the number of macroprolactinemia patients. Future prospects for hyperprolactinemia may require diagnostic criteria using free prolactin levels and so MPRL fraction measurement is important for the diagnosis and treatment of patients with obstetric and gynecological symptoms. HYPERPROLACTINEMIA is one of the most common hypothalamic pituitary endocrine disorders [1][2][3]. The causes of hyperprolactinemia include pregnancy, prolactinoma, intracranial tumors, medication, and hypothyroidism [1], and 9%-39% of hyperprolactinemia cases are classified as idiopathic [4,5]. On the other hand, Sapin et al. reported that macroprolactin (MPRL) fraction accounts for 31.6% of total PRL in hyperprolactinemia patients with a MPRL fraction in the normal range [6]. In many cases, MPRL fraction may be affecting the prolactin level and the diagnosis of hyperprolactinemia. Human PRL is heterogeneous in molecular size: the major circulating form is little PRL (MW 23 kDa), Submitted Aug. 14, 2017; Accepted Oct. 24, 2017 as EJ17-0358 Released online in J-STAGE as advance publication Nov. 22, 2017 Correspondence to: Kohzo Aisaka, Department of Obstetrics and Gynecology, Hamada Hospital, 2-5 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan. E-mail: aisaka-k@umin.ac.jp the remainder consisting of big PRL (MW 50 kDa), and big-big PRL (MW greater than 150 kDa) [7,8]. The monomeric isoform accounts for 80%-95% of PRL and is known to be both biologically and immunologically active in vivo [9]. Macroprolactinemia is a condition in which big-big PRL is substantially increased in the serum, and is usually diagnosed when the proportion of polyethylene glycol (PEG)-induced precipitable PRL exceeds 60% of the total PRL ...