There is much evidence of the importance of the preclinical phase of multiple sclerosis (MS). However, apart from a recent report of the incidental discovery of a case of primary progressive MS, there are no data on the sequential magnetic resonance imaging (MRI) follow-up of patients before the clinical phase. We report the incidental discovery of white matter changes on MRI and their follow-up in a patient three years before the first neurological event (optic neuritis). A 34-year-old woman presented with headaches and depression after her young daughter had been involved in a car accident and spent two weeks in intensive care. The woman's general practitioner performed a brain MRI, which revealed multiple T2-weighted hypersignals suggesting MS. During the next three years, clinical examination remained normal but we observed new T2 lesions and/or new enhanced T1 lesions after gadolinium infusion on the four successive MRIs. Thirty-seven months after the first MRI, the patient developed a right optic neuritis. The diagnosis of MS was made according to space and time dissemination on MRI criteria. We proposed a treatment with Interferon Beta 1a (Avonex).