In this report, we describe the case of a young, diabetic girl with ketoacidosis who suffered sudden loss of vision of the right eye. The loss of vision was caused by an invasive rhino-orbital-cerebral fungal infection (mucormycosis) with extensive periorbital thrombosis. Despite maximal antifungal and surgical treatment (including exenteration of the right orbit), the clinical situation deteriorated. It was only after overcoming the difficulties of managing her hyperglycaemia that the patient's condition stabilised and her life was saved. Another factor contributing to this girls' survival was the swift diagnosis of mucormycosis, which was made soon after the onset of symptoms. Because of this, treatment could be started almost immediately.