We compared the number of new gadolinium-enhancing and T2-weighted lesions on six monthly MRI scans in eight patients with relapsing multiple sclerosis before and during treatment with weekly intramuscular interferon beta-1a (Avonex; 30 microg). MRI activity was modestly reduced during treatment (p = 0.016). The treatment effect was also significant after adjusting pretreatment lesion frequency for regression to the mean. Based on this pilot study, Avonex, as approved for treatment of relapsing forms of MS in the United States, reduces new MRI activity.