Familial hypocalciuric hypercalcemia is caused by heterozygous loss-of-function mutation of the calcium sensing receptor gene, and this is characterized by mild, persistently elevated levels of serum calcium without symptoms or complications. We present a case of clinically diagnosed familial hypocalciuric hypercalcemia with unexpected low bone mass. A 19-year-old man presented with incidentally discovered hypercalcemia. He showed normal growth and sexual maturation. Biochemical studies showed hypercalcemia, increased parathyroid hormone, hypocalciuria, a decreased urinary calcium-creatinine ratio and decreased serum 25-hydroxy-vitamin D. The other hormonal studies were normal. Dual energy x-ray absorptiometry showed low bone mineral density, and the Sestamibi scan showed no abnormality in the parathyroid glands. Iliac bone biopsy showed a general decrease in bone density and increased porosity of the cortical bone. Normal mineralization was also shown, but in part, osteoid deposition was also found. Direct sequencing of the patient's calcium sensing receptor gene showed a point mutation at exon7, Q926R.