Case reports on 2 patients with arachnoidal cyst of the cisterna ambiens are presented. The first patients disorder was initially diagnosed clinically as parasellar tumor and later after pneumoventri-culography, as arachnoidal cyst of the cisterna ambiens, which was confirmed by postmortem examination. The second cyst was diagnosed by clinical findings and air study and cured by ventricular taps. This rare condition is difficult to diagnose. However, the combination of neurologic and ventriculographic findings should suggest it. Air study should be done with great care to avoid sudden change in intracranial pressure.