With increasing availability of magnetic resonance imaging (MRI), there is also an increase in incidental abnormal findings. MRI findings suggestive of multiple sclerosis in persons without typical multiple sclerosis symptoms and with normal neurological findings are defined as radiologically isolated syndrome (RIS). Half of the persons with RIS have their initial MRI because of headache, and some have a subclinical cognitive impairment similar to that seen in multiple sclerosis. Radiological measurements also show a similarity between RIS and multiple sclerosis. Approximately two-thirds of persons with RIS show radiological progression and one-third develop neurological symptoms during mean follow-up times of up to five years. Cervical cord lesions are important predictors of clinical conversion. Management has to be individualised, but initiation of disease modifying therapy is controversial and not recommended outside of clinical trials since its effects have not been studied in RIS. Future studies should try to establish the prevalence and long-term prognosis of RIS, its impact on quality of life, and define the role of disease modifying therapy in RIS.