Adefovir dipivoxil (ADV) is one of the commonly used antiviral agents in the treatment of chronic hepatitis B infection. Nephrotoxicity is dose related and occurred at a daily dosage of >30 mg. However, it is now increasingly recognized that nephrotoxicity can occur at a daily dose of 10 mg. We present a case of acquired Fanconi syndrome in a patient with chronic hepatitis B who had been treated with ADV for 4 years. She presented with progressive muscle weakness and generalized bone pain. The laboratory results showed the feature of proximal renal tubule dysfunction, particularly severe hypophosphatemia. Diagnostic approach to hypophosphatemia and proximal renal tubular dysfunction is discussed. After switching over from ADV to entecavir, her symptoms and laboratory findings returned to normal. Acquired Fanconi syndrome can be associated with ADV at a conventional dosage, and therefore, patients treated with long-term ADV should have regular monitoring of renal function and calcium and phosphate levels.