Prospective monthly magnetic resonance imaging (MRI) studies were done over 6 months in seven relapsing MS patients. MRI and neurologic evaluations were compared for sensitivity in detecting disease activity. Four patients were clinically stable throughout the study. Three patients had five clinical relapses, two localized to the spinal cord and three to the brainstem. Eighteen new and ten enlarging MRI lesions were seen in five patients. Most lesions were less than 10 mm in diameter. All were clinically silent. Two patients developed major enlarging MRI lesions (seen in three slices) which increased in size over 2 months and then gradually became smaller over 2 months, leaving behind small residual areas of abnormality. There were 36 follow-up scans, 17 of which (47%) showed evidence for increasing activity. Thirteen (36%) of the scans had new lesions, most of them being small. This study shows that MRI evidence for disease activity in MS is much more frequent than is clinical evidence.