In this study, we investigated the relationship between the concentrations of intact parathyroid hormone (i-PTH) and midregion PTH (m-PTH) measured by an immunoradiometric assay and a radioimmunoassay, respectively, versus various demographic and biochemical parameters, bone mineral density (BMD) of the lumbar spine (LS) and radius, and the radiographic findings of osteosclerosis and aortic calcification in hemodialysis (HD) patients. m-PTH correlated positively and more significantly with serum calcium (Ca), serum phosphorus (P), Ca-P solubility products (Ca x P) and LS-BMD than i-PTH did (P = 0.024 vs. 0.531, 0.001 vs. 0.061, 0.0001 vs. 0.125, and 0.017 vs. 0.284, respectively). A positive correlation between the percent changes in serum P over the 1-month measurement period and those in m-PTH rather than in i-PTH was also observed (P = 0.021 vs. 0.869). These data indicate than m-PTH is distinct from i-PTH in its positive correlation with serum Ca, serum P, Ca [symbol: see text] P and LS-BMD in HD patients. Since m-PTH is known to consist mostly of the midregion and carboxyl-terminal fragments of PTH in HD patients, the present study suggests that these PTH fragments may be biologically significant in the patients in vivo.