A 41-year-old male with a relapsing - remitting form of multiple sclerosis was followed-up by neurologists for incidental occurrence of a large temporo-fronto-parietal arachnoid cyst in the left hemisphere with a pronounced contralateral midline shift. Severe trigeminal neuralgia (lower two branches) on the right side and residual bilateral optic neuritis were present. The underlying diagnosis of MS, stationary behaviour of the arachnoid cyst, stable midline shift, and refusal of an operation by the patient decided for a non-surgical approach. Reasons for this decision are discus- sed.

        Key words: magnetic resonance imaging - multiple sclerosis - arachnoid cyst - trigeminal neuralgia