We describe an 81-year-old woman presenting with sudden onset of generalised chorea. She was unaware of suffering from diabetes. Laboratory screening revealed non-ketotic hyperglycaemia. Brain magnetic resonance imaging (MRI) failed to show basal ganglia abnormalities. Monotherapy with subcutaneous regular insulin induced a progressive normalisation of glycaemia as well as a parallel improvement of the abnormal involuntary movement scale on a nine-day sequential observation. This correlation strongly supports the hypothesis that non-ketotic hyperglycaemia itself might play a major pathogenetic role in chorea associated with non-ketotic hyperglycaemia. Diabetes mellitus should be suspected in patients who develop sudden onset of chorea even in the absence of putaminal abnormalities on MRI.