Background: Monofocal acute inflammatory demyelinating (MAID) lesions present as large masses that mimic brain tumors. The natural history and nosologic classification are not well defined.

Methods: Five patients with MAID from our files and 4 from the literature are presented. We defined MAID as a mass-like lesion, >4 cm in diameter. Demyelination was pathologically proven in all patients.

Results: Headaches and motor deficits were the most common presenting symptoms. Language problems and motor deficits were the most common findings on physical examination. After a median follow-up of 44 months, none of our 5 patients had multiple sclerosis. In similar cases, additional lesions indicative of multiple sclerosis developed within an average of 8 months.

Conclusions: It may be difficult to distinguish MAID from brain tumor. It may be that MAID represents a unique form of isolated demyelinating disease, or it may represent transition to multiple sclerosis.