Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system, which often follows a relapsing-remitting (RR) course with discrete attacks. MS attacks have been associated with upper respiratory infections (URIs), but the specific viruses responsible have not been identified. We studied a cohort of 16 RRMS patients experiencing URI and followed them for clinically identifiable attacks. The viral causes of 21 separate URIs were investigated using culture and polymerase chain reaction (PCR) of nasal swab specimens, and by serology. Sibleys 'at-risk' period for MS attacks, beginning two weeks before and continuing for five weeks after a URI, was used for the analysis. Seven of the nine (78%) URIs due to picornaviruses were associated with an MS attack during the at-risk period. By contrast, only two of 12 (17%) picornavirus-negative URIs were associated with an MS attack (P = 0.01). The possible role of picornaviruses in the pathogenesis of MS deserves further study.