We encountered a 16-year-old boy with Japanese Dent's disease who exhibited renal insufficiency and an osseous disorder of the spine. Proteinuria first was noted at the age of 2 years. At 13 years, the patient underwent analysis of the CLCN5 gene, which identified missense mutation (I524K) in exon 10. During follow-up, a marked increase in urinary beta2-microglobulin was associated with mild deterioration of renal function. At the age of 15 years, hypocalcemia (7.5 mg/dl) accompanied by an increased plasma concentration of alkaline phosphatase was first detected. At that time, plasma concentration of 25(OH)D3 and 1'alpha25(OH)2D3 were low accompanied by a high plasma parathyroid hormone concentration. A renal biopsy specimen revealed tubulointerstitial alterations including mononuclear cell infiltration, partial fibrosis and focal glomerular sclerosis. Immunofluorescence revealed weak, discontinuous staining of megalin along the brushborder of renal proximal tubules. Western blotting demonstrated decreased urinary excretion of megalin. Thus, clinical manifestations and prognosis may vary in Japanese Dent's disease. Reduced megalin expression may have disturbed calcium homeostasis, leading to osseous disorder in our patient.