Abstract： Objective To specify fenestration site for arachnoid cysts in cisterna magna according to CSF dynamics by reviewing these neuroendoscopically treated cases in our department.Methods According to lesion locations and CSF dynamics,twenty - seven cases surgically indicated were fenestrated to either of quadrigeminal cistern and supracerebellar cistern.Clinical and radiological relief were followed after operation as well as discharge.Results A case of Mega cisterna magna was misdiagnosed as arachnoid cysts.Quadrigeminal cistern fenestration was manipulated in 6 cases,supracerebellar cistern fenestration in the other.Average period for follow - up was 14.5 months,88％ ( 23/26 ) of cases was relieved symptomatically,92％ (24/26) reverted to negative Romberg sign,and 8％ (2/26) deflated in cyst volume.Meanwhile four cases of obstructive hydrocephualus were normal in ventriclar shapes after third ventriculotomy treatment.Conclusions CSF- orientated fenestration for arachnoid cysts in cisterua magna has shown promising effects clinically.As a result,physiological pattern of CSF - circulation should be restored primarily for it.