A case of a 51-year-old woman with reversible cerebral vasoconstriction syndrome (RCVS) without an associative cause is reported. Initially the diagnosis primary angiitis of the central nervous system (PACNS) was considered. Both diagnosis are rare and can mimic each other. Distinction between both can be difficult, but is necessary because of different treatment options. Clinical features and diagnostic assessments to distinguish RCVS from PACNS and the potential pitfalls are discussed.