Hyperglycemia may present with polyuria and polydipsia, which are also the main clinical features of diabetes insipidus (DI). Diabetic patients with polyuria and polydipsia may be mistaken as poor glycemic control, which will delay the diagnosis and treatment of DI and related underlying disease. Herein, we present a 54-year-old man who had polyuria and polydipsia for three months. Initial laboratory data showed HbA1C 7.9%, and urine glucose 1000 mg/dL. Diabetes mellitus was the initial diagnosis and treated with metformin. However, polyuria and polydipsia persisted even though hyperglycemia had been kept under control. Low urine osmolality and high urine output led to the diagnosis of DI. A desmopressin test confirmed the presence of central DI. After a series of study, pituitary metastasis of lung cancer (adenocarcinoma) was the final diagnosis. The patient’s polyuria and polydipsia improved after treatment with intranasal desmopressin; while multiple brain metastases were treated with CyberKnife® radiosurgery. In conclusion, the development of DI with polyuria and polydipsia in diabetic patients with poor glycemic control could be overlooked, which will delay the diagnosis and treatment of underlying disease such as malignancy. Monitoring urine osmolality or specific gravity for diabetic patients with significant polyuria and polydipsia is helpful. (Acta Nephrologica 2011; 25: 137-140)