Posterior reversible encephalopathy syndrome (PRES) is a reversible leukoencephalopathy characterised by subcortical vasogenic oedema and neurological signs. We present the case of a 64-year-old woman who presented to hospital with symptomatic primary hyperparathyroidism. Her parathyroid hormone (PTH) level on admission was elevated at 1,330ng/l (normal range15-68ng/l) and her serum calcium measured 4.83mmol/l (normal range 2.25-2.54mmol/l). Technectium-99m sestamibi scan demonstrated a focus of radiotracer uptake consistent with a right upper parathyroid adenoma or carcinoma. After commencing appropriate medical treatment, the patient developed intractable seizures necessitating endotracheal intubation. Magnetic resonance imaging of her brain revealed bilateral symmetrical T2 hyperintensities in the posterior circulation consistent with PRES. Following stabilisation and further medical treatment for hypercalcaemia, the patient underwent a parathyroidectomy. Preoperative rapid PTH assay measured 1,021ng/l. Following excision, PTH levels fell to just 10ng/l. She was extubated in the intensive care unit on postoperative day 1 and made an uneventful recovery. At her 6-week follow-up appointment, all neurological symptoms had resolved. PRES is a rare neurological entity more often seen in the setting of hypertension, immunosuppression and renal failure. The development of new neurological manifestations in the setting of known risk factors should raise suspicion for the underlying diagnosis.