Secondary hyperparathyroidism is one of the severe complications of chronic renal failure. In this study, we investigated the cellular components of parathyroid tissue, with measurements of various serum parathyroid hormone (PTH) types in the circulation, and evaluated their clinical significance in hemodialysis patients. Thirty-eight patients who underwent both subtotal parathyroidectomy and autotransplantation in Tokai University Hospital from 1979 to 1994 were divided into two groups. Group 1 (G-1) was not administered active-type vitamin D<sub>3</sub> (VD; 19 patients; 15 males and 4 females), and group 2 (G-2) was administered VD (19 patients; 10 males and 9 females). The parathyroid tissues which were obtained from G-1 and G-2 patients were classified into three classes based on the size of the oxyphilic cell area (class I < 25%, class II25-50% and class III > 50%). Our results showed that the proliferation index of parathyroid tissues in the oxyphilic cell area was higher in G-2 than that in G-1. Immunohistochemically, MIB-1 staining was more intense than that in the chief cell area in G-2 patients. Moreover, the proliferative index in the same specimen was also higher in the oxyphilic cell area than in the chief cell area. It was suggested that the oxyphilic cells proliferated independently. Furthermore, synthesis of PTH in the oxyphilic cell area was revealed immunohistochemically by the presence of PTH and was confirmed by positive staining of PTH mRNA in the oxyphilic cell area with in situ hybridization. HS(M-terminal)- and C-PTH levels in the serum were significantly higher in class III than in class I (p < 0.01). No significant difference of HS- and C-PTH levels between class II and class III was noted. Moreover, no significant difference of intact-PTH levels was found in all three classes. From the above findings, it was suggested that proliferation of the parathyroid tissues or secretory state of PTH in hemodialysis-maintained patients with secondary hyperparathyroidism, which was closely related to the proliferation of oxyphilic cells, can be more accurately reflected by HS- and C-PTH levels than by the intact-PTH level. Therefore, it was suggested that HS- and C-PTH levels in the serum are important indices for accurate evaluation of the pathology and suitable therapy of secondary hyperparathyroidism, as well as observation of the clinical course.