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Abstract We report a patient with renal tubulointerstitial fibrosis and symptomatic osteomalacia associated with Fanconi syndrome. A 55-year-old woman was hospitalized because of an inability to walk. Beginning approximately 2 years previously, she had experienced gradually worsening pain in the hips, shoulders, and trunk, culminating in a bedridden state. Serum urea nitrogen was 38 mg/dl; creatinine, 2.6 mg/dl; uric acid. 3.6 mg/dl; phosphate, 2.3 mg/dl; and alkaline phosphatase, 2111 IU/l. Urinary β2 microglobulin was 72 331 μg/day. Aminoaciduria, renal glucosuria, and proximal renal tubular acidosis with a normal anion gap were also noted. The patient was diagnosed with Fanconi syndrome. Radiography demonstrated typical Looser zones in the proximal portion of the left and especially the right femoral shaft, and at several other sites. A renal biopsy specimen disclosed severe tubulointerstitial fibrosis with little cellular infiltration. Glomeruli were largely intact. A bone biopsy specimen indicated osteomalacia; no tetracycline labeling could be seen along most trabecular bone surfaces, and the ratio of total osteoid volume to bone volume was increased (71.8%). Bicarbonate administration (9 g/day) gradually lessened most symptoms, permitting ambulation. Calcitriol administration decreased excessive intact-parathyroid hormone emerging after 2 months of acidosis correction. Thus, severe acidosis associated with Fanconi syndrome can induce osteomalacia showing serious skeletal complications, but also responsiveness to bicarbonate therapy.