A 50-year-old man without family history of metabolic bone disease was referred to our hospital with a 5-year history of progressively worsening spinal and bilateral diffuse leg pain and proximal muscle weakness. Two years before admission, he was diagnosed as ankylosing spondylitis by a rheumatologist and was maintained on low-dose prednisone therapy without benefit. He developed progressive spinal and thoracic deformities, resulting in a 10 cm loss in height in the preceding 2 years. On physical examination, marked thoracic kyphosis and pectus carinatum was noted. Plain radiograph revealed pseudofracture in the right femoral neck. Laboratory findings showed a normal level of serum calcium, elevated level of serum alkaline phosphatase and inappropriately increased urinary phosphate excretion despite extreme hypophosphatemia. He was diagnosed as adult-onset hypophosphatemic osteomalacia caused by renal phosphate wasting. Serum fibroblast growth factor 23 was the upper limit of normal despite extreme hypophosphatemia and no neoplastic lesion potentially inducing hypophosphatemic osteomalacia could be identified in a thorough search including imaging studies of his entire body. Oral administration of phosphate and activated vitamin D together with dipyridamole relieved the persistent pain and weakness, and he became fully ambulatory.