As short-term MRI studies are increasingly being used to monitor disease activity in multiple sclerosis (MS) it is vital to establish if short-term MRI activity is predictive of long term clinical outcome. We followed up after 5 years a group of 10 benign (relapsing-remitting MS with a disease duration > 10 years and EDSS < or = 3) and 10 early relapsing-remitting patients who previously had monthly serial MRI scans for 6 months. In the early relapsing-remitting group median EDSS at entry to the initial serial study was three and in the benign group 2.5. At 5-year follow up, five of these 20 patients had developed a definite deterioration in EDSS. The median number of new enhancing lesions detected originally in the group that had deteriorated was 11 (7-17) compared to 0 (0-5) new enhancing lesions, for those who had not deteriorated (P < 0.05). There was a trend towards a higher baseline T2 lesion load in the group with a definite change in EDSS but this was not significant This study suggests that short-term measurement of the number of gadolinium enhancing lesions may predict long term outcome in relapsing-remitting MS.