Primary osteoporosis is rare in children and adolescents and its optimal pharmacological management is uncertain. Bisphosphonates are commonly used while denosumab has only been administered to a few children with osteogenesis imperfecta. We studied a treatment-naïve 13.5-year-old boy with severe osteoporosis and multiple vertebral deformities who presented with back pain and difficulty in walking. Causes of secondary osteoporosis were excluded and there were no abnormalities in genes known to cause bone fragility. He was treated with denosumab 60 mg subcutaneously every 3 months for 30 months, and he was pain-free within 6 weeks after the first injection. Lumbar spine BMD and femoral neck BMD increased with treatment by 65.6% and 25.3%, respectively, and deformed vertebrae regained their normal shape; linear growth was not impaired. During the second year of treatment, transient hypercalcemia (maximum 3.09 mmol/l) before the denosumab injection was observed. In conclusion, denosumab was highly effective in this case of primary pediatric osteoporosis, with remarkable clinical and radiological response. Transient hypercalcemia was probably due to amplification of the effect of growth spurt and puberty on bone remodeling by the transient, short-term discontinuation of the drug. Furthermore, our data suggest that mobilization of calcium from treatment-induced sclerotic transverse lines in bone metaphyses may contribute to the development of hypercalcemia.