Forty-eight chronic hemodialysis patients were divided comparably into two groups (24 patients in each). Two micrograms of 1 alpha(OH)D3 was administered to one group for 3 months and its placebo to the other group. In the 1 alpha(OH)D3-treated group, serum total calcium increased from 7.82 +/- 0.11 (mean +/- SEM) to 9.70 +/- 0.27 mg/dl (p less than 0.001) which was significantly higher (p less than 0.001) than control values of 8.86 +/- 0.06 mg/dl from normal volunteers. Systolic, diastolic and mean blood pressures, however, did not change significantly throughout the study. Even in the 9 patients who had a substantial increment of serum calcium of more than 2 mg/dl with hypercalcemia (greater than 10 mg/dl) at 3 months, no significant changes in blood pressure were found. Serum iPTH decreased from 2.83 +/- 0.28 to 0.98 +/- 0.23 ng/ml (p less than 0.001) at 3 months of treatment. Furthermore, a significant inverse correlation was obtained between the changes in serum calcium and iPTH. In the placebo group there were no significant changes in serum calcium, iPTH and blood pressure during the 3-month treatment period. The present study indicates that a substantial increase in serum calcium or a chronic hypercalcemia induced by 1 alpha(OH)D3 treatment in maintenance hemodialysis patients does not accompany a rise in blood pressure, probably due to a concomitant suppression of PTH. The results also suggest that the hypocalcemic state found in hemodialysis patients is not associated with any significant change in blood pressure. The importance of PTH in blood pressure regulation was discussed.