Seventeen patients (9 men, 8 women; aged 27 to 75 years) who were on chronic hemodialysis for 1 to 14 years were included in the study because they had severe hyperparathyroidism diagnosed by elevated plasma alkaline phosphatase and on plasma intact PTH levels more than twice the upper limit of normal. They had been previously treated with various combinations of oral calcium and/or Al(OH)3 as phosphate binders, oral 1 alpha(OH) vitamin D3 metabolites and a dialysate calcium concentration (DCa) of 1.6 to 1.75 mmol/liter. When i.v. alpha calcidol was introduced DCa was reduced to 1.25 mmol/liter and CaCO3 taken with the meal was used as the sole phosphate binder. alpha calcidol was i.v. injected after the third dialysis of the week at a dose up to 4 micrograms per dialysis in order to obtain a predialysis plasma concentration of Ca at 2.5 +/- 0.2 and PO4 between 1.5 and 2 mmol/liter. All the other treatments were discontinued. During the six months of follow-up, the mean weekly dose of alpha calcidol was 6 micrograms and CaCO3 700 +/- 50 mmol. Plasma calcium (PCa) increased moderately from 2.35 to 2.47 mmol/liter (P < 0.05) whereas plasma PO4 (PPO4) did not significantly increase (1.56/1.64 mmol/liter). Total alkaline phosphatase and its bone isoenzyme activity decreased significantly to normal values [respectively from 186 to 83 IU (normal: 135) and from 102 to 32 IU (normal < 33)] whereas plasma intact PTH decreased from 485 to 125 pg/ml (normal < 55).(ABSTRACT TRUNCATED AT 250 WORDS)