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OBJECTIVE This study clarifies the incidence of prolactin-producing pituitary adenomas without hyperprolactinemia and determines the appropriate treatment strategy for these tumors. PATIENTS AND METHODS This retrospective analysis focused on prolactin-producing adenomas without hyperprolactinemia, which were initially treated by surgery as nonfunctioning pituitary adenomas. Among 942 patients with histologically confirmed pituitary adenoma, 114 (12.1 %) patients, consisting of 68 men and 46 women, who had prolactin-producing adenomas without hyperprolactinemia were identified between April 2005 and March 2019. RESULTS Of the 114 patients identified, 13 (11.4 %) had prolactin mono-expressions, 18 (15.8 %) had pit-1 lineage hormonal expressions, and 83 (72.8 %) had paradoxical immunoexpression out of the pituitary differentiation lineage, including prolactin. During the follow-up period, 19 patients suffered tumor progression, and 14 required salvage treatment. Of the 19 patients, 11 underwent gamma knife radiosurgery, and none of them experienced further tumor progression. Cabergoline was administered of them to six patients, and one achieved tumor shrinkage. However, the remaining five patients who were treated with cabergoline suffered further tumor progression and required another salvage treatment. Among the patients in the prolactin mono-expression group, one experienced tumor regrowth and underwent gamma knife radiosurgery. In the pit-1 lineage group, two patients experienced tumor regrowth. One had further tumor progression after treatment with cabergoline and underwent gamma knife radiosurgery. Among the patients in the paradoxical immunoexpression group, 16 suffered tumor progression. Four patients underwent further surgery, seven patients were treated with gamma knife radiosurgery, and one patient received fractionated irradiation. None of the eight patients who were treated with gamma knife radiosurgery and fractionated irradiation showed further tumor progression. Four patients in this group were treated with cabergoline, but they all suffered further tumor progression and underwent additional salvage treatments. CONCLUSIONS Out of the pituitary differentiation lineage, paradoxical hormonal expression occurred in three-quarters of the patients identified. Further surgery or gamma knife radiosurgery should be given priority in times of tumor progression because most patients were resistant to dopamine agonists.