In 2003, a 64-year-old woman was diagnosed with mixed connective tissue disease and treated with oral prednisolone (30 mg/day). The prednisolone dose was gradually decreased, and a dose of 5 mg/day had been maintained since 2004. In 2009, she gradually developed vision loss, malaise, anorexia, and throat pain due to hydrodipsia. She was noted to have iritis and vitreous opacity by an ophthalmologist, and was referred for further evaluation. Fine rales were audible throughout the entire lung field, and chest CT showed diffuse small nodules that were more prominent on the upper and middle lobes, and swelling of the mediastinal and hilar lymph nodes. Transbronchial lung biopsy showed many epithelioid granulomas with multinuclear giant cells, compatible with sarcoidosis. Polyuria was identified as a cause of hydrodipsia and a diagnosis of partial central diabetes insipidus was made. High-dose prednisolone (40 mg/day) together with intranasal administration of desmopressin resulted in improvement of all of her clinical symptoms. MCTD followed by sarcoidosis is rare. Furthermore, this is the first reported case of MCTD complicated by sarcoidosis and central diabetes insipidus.