An anatomical and radiological study of seven cases with arachnoid cyst of the midline of the posterior fossa in which there was either necropsic or surgical anatomical diagnosis is reported. A great variation was observed regarding their location, size and extension, which covers all the zone existing between the foramen of Magendie and the quadrigeminal lamina, presenting two cases which rose to the medial fossa, one of them entering in a left lateral ventricle and implanting itself in its' walls. Associated to arachnoid cysts other malformations have been found, such as type I Chiari, partial agenesis of the vermis, cerebellar dismorphies and stenosis of the aqueduct. Neuroradiological examination is fundamental for diagnosis, existing important alterations both in plain skull X-Rays and pneumoencephalography, ventriculography and vertebral and carotid angiographies. There are no strict neuroradiological patterns in the study with air nor in arterial and or venous times of angiography which define assuredly, presence, of the arachnoid cyst, since each case shows different findings. However, the conjugated analysis of all explorations must give correct diagnosis in almost the totality of cases.