The long-term safety and efficacy of synthetic 1,25-(OH)2D3 (calcitriol; Rocaltrol) in the treatment of women with type 1 osteoporosis is being assessed in a randomized trial. Patients were allocated in double-blind fashion to 1,25-(OH)2D3 or matching placebo. Initially, the calcium intake was adjusted to 1,000 mg/d. The study protocol called for increasing the dose of 1,25-(OH)2D3 until patients developed either hypercalcemia or hypercalciuria. However, in order to maintain a higher dose of calcitriol on a long-term basis, the calcium intake had to be reduced to 600 mg/d in those receiving calcitriol; if that was not successful in eliminating hypercalcemia and hypercalciuria, then the dose of 1,25-(OH)2D3 was reduced as necessary. During the hypercalcemic phase, the indices of bone resorption decreased significantly, demonstrating that calcium absorption is solely responsible for hypercalcemia. The maintenance dose was established after 8 to 10 weeks, and the 24-hour urine calcium and creatinine clearance remained constant throughout the remainder of the study period. On a calcium intake of 600 mg/d, the long-term maintenance dose of 1,25-(OH)2D3 averaged 0.675 micrograms/d. Long-term therapy on an average dose of 0.675 micrograms/d was not associated with nephrotoxicity.