A 38-year-old woman with diabetes mellitus complained of acute visual loss in the left eye (20/200) and swollen left upper eyelid. Slit lamp examination of the left eye revealed ciliary injection, posterior synechia iritis, numerous inflammatory cells, and fibrin exudates in the anterior chamber. T1-weighted enhanced magnetic resonance imaging demonstrated left lacrimal gland enhancement with inflammatory spread to the left anterior ocular segment. Blood examination showed increased blood sugar but the other components were within normal limits. The patient was treated with steroid pulse therapy (methylprednisolone 1 g/day × 3 days) under a blood sugar control regimen in consultation with an endocrinologist, after which additional peribulbar injection of triamcinolone acetonide (40 mg) was performed. Resolution of the anterior uveitis and the dacryoadenitis was obtained after 2 months and there was no recurrence 1 year after the therapy. This is a rare case of severe anterior uveitis caused by idiopathic dacryoadenitis in a patient with diabetes mellitus.