A 51-year-old female with chronic hepatitis B treated by Lamivudine (LAM) and Adefovir Dipivoxil (ADV) because of breakthrough hepatitis developed hypophosphatemia. She was diagnosed with Fanconi’s syndrome caused by the long-term administration of ADV. Since her symptoms did not improve after decreasing the ADV dosage and administering vitamin D, she was given L-carnitine. This led to a gradual improvement of her symptoms and the hypophosphatemia. These results indicate that a decrease in the ADV dose combined with supplementation with L-carnitine led to an improvement of Fanconi’s syndrome acquired by a patient with chronic hepatitis B while taking ADV.