In around 85% of cases, multiple sclerosis (MS) starts with an acute neurological episode, a clinically isolated syndrome, which is considered to be the first clinical episode of relapsing-remitting MS. It is characterised by the presence of acute relapses, after which there is normally good functional recovery. Investigations need to rule out conditions that can mimic an inflammatory-demyelinating disease of the CNS and determine the presence of dissemination in space and dissemination in time of the inflammatory-demyelinating disease. There is no confirmatory test for MS and it remains essentially a clinical diagnosis.