Paricalcitol was introduced recently as an effective alternative to calcitriol for the suppression of parathyroid hormone (PTH) in patients with end-stage renal disease. An international, multicenter, double-blinded, randomized, comparative study of intravenous paricalcitol and calcitriol was performed. Results from 38 patients at dialysis units affiliated with the Northwestern University Medical School (Chicago and Evanston, IL) are reported here while a report of the full clinical trial is being completed. Results were evaluated in terms of obtaining the following end points: decrease of at least 50% in baseline PTH concentration and the occurrence of hypercalcemia and hyperphosphatemia. Paricalcitol therapy was started at a dose of 0.04 microgram/kg and increased by 0.04 microgram/kg increments every 4 weeks to a maximum allowable dose of 0.24 microgram/kg or until there was at least a 50% decrease in serum PTH concentration. Calcitriol therapy was started at a dose of 0.01 microgram/kg and increased by 0.01 microgram/kg increments every 4 weeks to a maximum allowable dose of 0.06 microgram/kg or until there was at least a 50% decrease in serum PTH concentration. Mean baseline serum PTH, calcium, and phosphorus concentrations were similar. Reductions in PTH occurred more rapidly in subjects administered paricalcitol compared with calcitriol, with no difference in serum calcium levels throughout the study between groups. The percentage of subjects experiencing severe hyperphosphatemia (serum phosphorus >8.0 mg/dL) was greater in those administered calcitriol compared with paricalcitol. In conclusion, our data suggest that paricalcitol reduces PTH levels more rapidly, with fewer episodes of hyperphosphatemia, than intravenous calcitriol.