A 14-year-old female with insulin dependent diabetes mellitus for 3 months, suddenly developed - in less than one day - a dense 'snow-flake cataract' with severe visual impairment in the left eye. In hospital, 5 days later one morning, the patient experienced additional loss of vision in the right eye due to further rapid cataract development, in close relation to hypoglycaemia in the early morning. In another case, a 22-year-old diabetic woman, treated with insulin for 1 year, developed an acute diabetic cataract in less than one day with pronounced visual reduction. Previously, only a few cases of acutely developed and irreversible diabetic cataract have been published. The prevalence of this rare type of diabetic cataract is estimated to be below 1% among diabetics.