This is the only confirmed case of St Louis Encephalitis reported in the UK in 2011. The St Louis virus is a member of the single stranded RNA Flaviviridae family and is transmitted to humans by mosquitoes, with birds acting as the amplifying host. The viraemia terminates approximately 1 week after the initial infection with subsequent neuronal damage being mediated immunologically. Incidence in the UK is extremely rare and approximately 10 cases occur every year in the USA where it is endemic. A 77-year-old man presented to 1 week after returning from a 7 week stay in Jamaica, with collapse, confusion and ataxia. Half way through his stay in Jamaica his relatives noticed that he had developed slurred speech, drowsiness and double incontinence. His mobility and power had gradually declined leading to recurrent falls. On admission he was apyrexial, haemodynamically stable, confused with systemic examination unremarkable. Nervous system exam demonstrated pyramidal weakness in both upper limbs but normal power and reflexes in his lower limbs. On day 4 of his admission he developed complex partial status confirmed on EEG which was successfully treated with anticonvulsants and acyclovir. Following this he remained in a persistently drowsy but rousable state. Investigations revealed a rising titre of the St Louis virus IgG in the serum on serial testing and IgG antibodies in the CSF, confirming the diagnosis. In patients with encephalitis who have recently returned from the Americas, infection by the St Louis virus should always be considered.