The novel Aristaless related homeobox gene, ARX, is widely expressed in the brain and is thought to play a key role in the regulation of brain development. Neurological phenotypes caused by ARX mutations have recently started to unfold. We describe a 72 year old man with X-linked mental retardation due to a 24 bp duplication mutation in exon 2 of the ARX gene. Cerebral MRI showed bilateral cystic-like cavities in both the cerebral and cerebellar hemispheres. No retraction or expansion in neighbouring parenchyma was observed, there was no history of acute neurological impairment, and no risk factors for cerebrovascular disease were found. The lesions appeared to be congenital and represented benign developmental cysts, possibly caused by the ARX mutation.