Folie á deux is a rare mental syndrome, firstly described by Falret in France on 1997, and renamed in the DSM-IV as shared psychotic disorder, and in the ICD-10 as induced delusional disorder; in which a paranoid or delusional belief is shared by two people with a very close relationship, and usually belonging to the same family. Social withdrawal is a common characteristic of both persons during the initial stage of the disorder.

A 16 years-old adolescent boy, born in Russia, and having this syndrome is presented in this study. After his arrival to Spain, he lived with his father with important social isolation for six months, and he begun to report delusional ideas with mystical-religious and ufological content. His father was diagnosed as having a delusional disorder with the same contents. Both patients received behavioural-cognitive therapy (CBT) with the aim of modifying their delusional beliefs, even that they went on living together. No antipsychotic medication was needed.

Clear changes were observed in their delusional system, particularly in the degree of conviction, as well as in distress, thought interference, and anxiety, and also an improvement was observed in the understanding of their beliefs, and in their social relationship with peers.

Social withdrawal as well as cultural differences associated to their immigrant condition played a critical role in the establishment of the induced delusional disorder, and CBT modified the delusional system, even father and son continued living together.