A young women presented with chronic headaches associated with a cyst of the right lateral ventricle. The diagnosis of intraventricular so-called "arachnoid" cyst was supported by CT scan, MRI and stereotactic puncture. MRI was of great value for demonstrating that the cyst was located within the lateral ventricule, that it was delineated by a thin wall adherent to the choroid plexus and that the cyst content was CSF-like.