Long-term dialysis for patients with end stage renal disease leads to an unavoidable common complication, which is secondary hyperparathyroidism. Two histological patterns (nodular and diffuse hyperplasia) are detected, indicating that continuous uremia-related stimulation promotes parathyroid cell proliferation from diffuse to nodular growth. However, the key molecular mechanism is not fully understood, which narrows the range of therapeutic options for advanced secondary hyperparathyroidism. To address this issue, we utilized surgically resected normal and hyperplastic parathyroid glands to perform immunohistochemical analysis of a multifunctional cell cycle modulator, CCAAT enhancer binding protein (C/EBP)β. In contrast to normal parathyroid tissue and diffuse hyperplasia, the intensity of C/EBPβ staining was homogeneously increased in the parathyroid cells from nodules, along with a higher cyclin D1 labeling index (108.0±19.5, mean±SEM) and Ki-67 labeling index (31.70±0.49). Normal and diffuse hyperplastic parathyroid glands had far fewer cyclin D1- and Ki-67-positive cells (P<0.001). Immunofluorescent double staining showed abundant coexpression of Th235 (mitogen-activated protein kinase [MAPK] phosphorylation site) C/EBPβ, along with upregulation of cytoplasmic Ras in nodular hyperplasia. In conclusion, hyperplastic parathyroid cells in nodules have an autonomous proliferation mechanism similar to that of cancer, in which C/EBPβ is upregulated and phosphorylated to interact with the oncogenic Ras/MAPK pathway. C/EBPβ may be a novel target molecule for blocking the growth circuit that underlies parathyroid tumorigenesis in secondary hyperparathyroidism. © 2011 The Authors. Therapeutic Apheresis and Dialysis © 2011 International Society for Apheresis.