Autosomal recessive osteopetrosis (OP) is characterized by insufficient osteoclast activity resulting in defective bone resorption and marked increase in skeletal mass and density. OP has been successfully treated with hematopoietic cell transplantation (HCT), secondary to engraftment of donor-derived functioning osteoclasts resulting in remodeling of bone and establishment of normal hematopoiesis. Although hypercalcemia is a common presenting feature of OP, it may be observed following HCT due to engraftment of osteoclasts differentiated from the hematopoietic precursors. To characterize hypercalcemia after HCT-who is at risk, onset, duration and response to treatment-we evaluated 15 patients with OP treated at the University of Minnesota from 2000 to 2009. Hypercalcemia, defined as any single calcium >11.0 mg/100 ml after the first transplant, was found in 40% of patients. Median onset of hypercalcemia was 23 days and the duration was 2-24 days. Hypercalcemia was more common in patients older than 2 years of age at the time of HCT. Treatment with hydration, furosemide and s.c. calcitonin resolved hypercalcemia and resulted in no severe adverse events. In conclusion, hypercalcemia is common in patients with OP within the first 4 weeks after HCT, and more likely in older patients. Isotonic saline, furosemide and s.c. calcitonin were well-tolerated and effective treatments in our study population.