We reported a 92-year-old woman with hyponatremia (117 mmol/l) occurring three days after the introduction of tramadol. Diagnosis of inappropriate antidiuretic hormone secretion was based on blood and urinary analysis and dosage of antidiuretic hormone. Natremia became normal after tramadol cessation and fluid restriction. Natremia must be measured when neurological abnormality occurs with tramadol treatment.