A 42-year-old female with Crohn's disease who had previously undergone multiple surgical interventions developed marked hypocalcemia, which could not be resolved with calcium administration. Markedly reduced serum magnesium levels were also observed. After intravenous magnesium administration, serum calcium levels rapidly normalized. In addition, the plasma levels of intact parathyroid hormone increased immediately after magnesium administration. These data strongly suggest that hypocalcemia resulted from disturbance of appropriate parathyroid hormone secretion caused by hypomagnesemia. After introduction of infliximab therapy, her abdominal symptoms and endoscopic findings improved, and serum calcium and magnesium levels stabilized within the normal range without magnesium administration.