This study was carried out to evaluate the acute effect of either 1.75 mmol/l calcium (A-HDF) or bicarbonate-containing and calcium-free (B-HDF) substitution fluid on calcium balance, serum ionised calcium, and intact parathyroid hormone (PTH) during haemodiafiltration (HDF: 1.3 m2 polysulphone dialyser; blood flow rate, 400 ml/min; treatment time 210 min; substitution fluid volume, 10 litres; dialysate calcium, 1.75 mmol/l). During A-HDF calcium balance was slightly positive (1.47 +/- 2.95 mmol), serum ionised calcium increased from 1.24 +/- 0.08 mmol/l to a final 1.46 +/- 0.05 mmol/l at the end of the treatment (P less than 0.001), and PTH decreased from 227 +/- 231 pg/ml to 150 +/- 123 pg/ml (P less than 0.01), during B-HDF calcium balance was negative (-3.06 +/- 1.3 mmol), ionised calcium decreased from 1.29 +/- 0.05 to 1.24 +/- 0.05 mmol/l (P less than 0.05) without inducing a statistically significant increase in PTH. However, in some patients the increase in PTH after treatment was greater than 50%. When the ultrafiltration rate was increased to 75-100 ml/min and bicarbonate-containing and calcium-free replacement fluid was used (B2-HDF), calcium balance became more negative (-9.75 +/- 4.55 mmol) and the decrease of ionised calcium was greater (from 1.3 +/- 0.06 to 1.14 +/- 0.04 mmol/l, P less than 0.01) inducing a significant increase of PTH (from 297 +/- 366 to 443 +/- 518 pg/ml, P less than 0.01). PTH changes were negatively correlated (r = -0.85) to ionised calcium changes during the treatment.(ABSTRACT TRUNCATED AT 250 WORDS)