A prospective study of serial magnetic resonance (MR) scans of the brain was carried out every 2 weeks for 4 to 6 months in 9 patients with mild, clinically definite, relapsing/remitting multiple sclerosis (MS). Six of the 9 patients developed a total of 12 asymptomatic new lesions in various parts of the brain. In none of the patients were the changes on MR scan accompanied by relevant new neurological symptoms or signs. New MR lesions had a characteristic temporal profile, reaching a maximum size in approximately 4 weeks before gradually shrinking, usually leaving a small residual abnormality indistinguishable from chronic MS lesions. The frequent occurrence of new asymptomatic lesions indicates that MS may be a more active process even in mildly affected asymptomatic patients than has been previously realized. The results emphasize the potential importance of using MR scanning to measure disease activity in laboratory studies of MS and in the assessment of treatment, particularly in asymptomatic patients in the early stages. We suggest that the expanding and contracting new lesions are the basic or primary lesion in MS, that the characteristic demyelinated plaque is represented by the small residual area that these lesions shrink down to, and that the typical collection of scattered white matter lesions in chronic MS may represent the accumulated residua of dozens or more of these active lesions occurring over many years.