OBJECTIVETo study the distribution of intracranial arachnoid cysts in a large and nonbiased patient population.METHODSOne hundred twenty-six patients with 132 arachnoid cysts were studied. Patients were consecutively referred to our department during a 10-year period from a well-defined geographical area with a stable population.RESULTSThe cysts had a strong predilection for the middle cranial fossa; 86 patients (65.2%) had cysts in this location. Of 106 cysts with clearly unilateral distribution, 64 were located on the left side and 42 on the right side. This significant difference resulted solely from the marked preponderance of middle fossa cysts for the left (left-to-right ratio, 2.1:1). There were significantly more males than females (92 males/34 females). This difference was exclusively due to male preponderance of unilateral middle fossa cysts (66 males/14 females; ratio, 4.7:1). For all other cyst locations, there was no difference between the two sexes (26 males/20 females) or the two sides (10 left, 16 right). The marked left-sidedness for middle fossa cysts was found only in males. Females had an even distribution between the two sides.CONCLUSIONArachnoid cysts have a strong predilection for the middle cranial fossa that may be explained by a meningeal maldevelopment theory: the arachnoid coverings of the temporal and frontal lobes fail to merge when the sylvian fissure is formed in early fetal life, thereby creating a noncommunicating fluid compartment entirely surrounded by arachnoid membranes. Why males develop more middle fossa cysts on the left side remains a mystery.