Arachnoid cysts situated in the middle cranial fossa constitute the largest group of this type of lesion. Their origin has been the subject of debate since they were first described. There is still controversy as to whether they originate directly from the meninges adjacent to the temporal pole or whether partial agenesis of the temporal lobe favours secondary formation of the cyst. We assessed the morphology of the temporal lobe and the bulging of the squamous temporal bone. "Paradoxical changes" in adjacent-bone, described as very rare findings in arachnoid cysts of the middle cranial fossa, were common in our study and suggest, in association with direct signs of temporal atrophy, that this atrophy precedes, or is at least co-existent with, the formation of the cyst. The importance of being able to determine the origin of the cyst lies in its being a factor to assess as regards treatment.