Central pontine myelinolysis (CPM) is a demyelinating disorder, which is associated most commonly with the rapid correction of hyponatraemia and other abrupt changes in physiological osmotic conditions. This includes the treatment of hyperosmolar hyperglycaemia in diabetes mellitus (DM) sufferers. Herein, we report a case of CPM in a 55-y-old patient with new-onset DM who presented with partial focal seizures and a sudden-onset right-sided hemiplegia. Magnetic resonance imaging revealed a lesion in the central pons. The patient responded to glucose control and antiepileptic medication, and achieved a recovery of limb function within 17 d of admission. CPM occurred in this patient before the correction of hyperglycaemic hyperosmolar state, and a disturbance in his initial electrolytes was not found. This report is the first documented case of long-term hyperglycaemic hyperosmolar state leading to CPM, and highlights that CPM can present as an isolated hemiplegia.