A 14 year-old boy presented, at a 3 month interval, with 2 episodes of hemiplegia of rapid onset. The first time, CT scan revealed a large parietal "tumor", shown to be pseudocystic at surgery, and histologically proven as a demyelinating disease. During the second attack, CT scan showed a similar but contralateral lesion, regressing completely under steroid treatment. 40 months after the onset of the disease, the child's life and intelligence are normal.