Treatment with methylprednisolone reduces the duration and severity of clinical relapses in multiple sclerosis (MS), while reducing the number of gadolinium-enhancing lesions on T1-weighted MRI. We performed serial MRI imaging after methylprednisolone treatment to see whether suppression of enhancement persists and whether related abnormalities on T2-weighted images disappear at follow-up. Thirteen patients with definite MS received a total of 31 courses of methylprednisolone over an average period of 50 weeks. Gadolinium-enhanced MRI was obtained before and after treatment, then at monthly intervals, using a standardised repositioning and imaging protocol. Two experienced readers in conference defined the number of active (gadolinium-enhancing and new or enlarging nonenhancing) lesions. We detected 609 active lesions on 195 examinations. Directly after treatment the reduction in the number of enhancing lesions was 78%, indicating restoration of the BBB and suppression of inflammation. It was uncommon for a lesion which stopped enhancing to show enhancement on a subsequent examination. No beneficial effect was observed on the rate of disappearance of related abnormalities on T2-weighted images, indicating persistent change such as oedema, cellular infiltration or demyelination. Moreover, in 89% of cases, an increase in the number of active lesions was observed before new clinical activity, if any, was observed (on average 52% earlier). MRI enabled us to demonstrate that the duration of the effect of methylprednisolone treatment is temporary (on average 9.7 weeks).