Hypomagnesaemia is common among hospitalised patients and is often under-recognised. Chronic alcohol abuse and alcohol withdrawal are known causes for severe hypomagnesaemia. Hypomagnesaemia can present with cardiac arrhythmias, seizures and other neurological symptoms, among which ataxia. We present a 57-year-old man with a history of chronic alcohol abuse who developed a subacute cerebellar syndrome with hypertension after alcohol withdrawal. A severe hypomagnesaemia of 0.19 mmol/L (normal values 0.70-1.10) was found. MRI showed diffuse, T2 hyperintense lesions in and swelling of the cerebellum. Symptoms, hypertension and MRI abnormalities significantly improved rapidly after intravenous magnesium supplementation. Hypomagnesaemia can cause a subacute, cerebellar syndrome and hypertension. Symptoms, hypertension and MRI abnormalities can be reversed with rapid magnesium supplementation. MRI abnormalities are similar to those caused by vascular endothelial dysregulation seen in posterior reversible encephalopathy syndrome (PRES). A similar case was recently described. We confirm that magnesium is likely to be involved in the pathophysiology of PRES.