24 patients with clinically isolated optic neuritis (ON) were examined with MRI. Only 5 patients (22.9%) had a normal MRI scan. The number of detected clinically silent lesions ranged from 0 to 38. They were mainly located in the frontal and parietal white matter. All patients with more than 3 lesions on MRI had pathological findings in CSF. There was no correlation between the number and location of lesions and age at onset of ON.