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Mega cisterna magna (MCM) is a cystic malformation of the posterior fossa and characterized by enlarged cisterna magna with an intact vermis and cerebellar hemispheres. The lesion is frequently seen as isolated however it may also constitute a part of Dandy-Walker complex. The concurrence of MCM and psychiatric disorders has already been documented but the possible pathophysiological associations still remain elusive. We present a 36 years-old male patient with incidentally detected isolated MCM, who previously had persecutory and reference delusions as a possible non-index case of shared psychotic disorder. Shared psychotic disorder (folie a deux) is a relatively rare mental condition develops between two affected individuals in a close relationship. In this entity, the index patient already has a psychotic disorder and transmits delusional beliefs on a healthy -secondary- individual. It is firmly suggested that abnormalities of cerebellar structures could involve the pathophysiology of psychotic disorders and based on this fact, we aimed to contribute to the regarding literature with discussing the predisposing role of cerebellar neurodevelopmental abnormalities in psychosis with presenting a possible shared psychosis case. [Anadolu Psikiyatri Derg 2018; 19(1.000): 106-109]