The first isolated case of folie à deux was reported by Harvey (1651). In 1873, Lasègue and Falret coined the term folie à deux or folie communiqué. Nowadays, defined as Shared Psychotic Disorder (DSM-IV-TR).

A case report of folie à deux involving two sisters, is described. The historical concept of shared psychotic disorder, its epidemiology, etiological hypothesis, clinical presentation and treatment is briefly reviewed.

A detailed clinical history was obtained from several interviews concerning the individuals taking part in this case; a literature review of the theme shortly surveyed.

A woman of 49 years, without any psychiatric history prior 2003, who developed hallucinatory activity and delusions of persecution concerning her neighbours, claiming they were attempting to test new technology using electromagnetic energy on her body. The patient constantly communicated the experience of these phenomena to her sister of 56, with whom she lived, during these 8 years. This second one, with schizotypical personality traits, accepted these delusions without question and, moreover, found proof of them in her own interpretations. Both patients were admitted to the inpatient ward and treated with antipsychotics.

Folie à deux is the transmission of delusional ideas from a psychotic to closely-associated individuals, who have experienced his domineering influence for a prolonged space of time. Most writers agree that prolonged social isolation, strong emotional ties, domination and forceful suggestive powers in one, coupled with a high degree of suggestibility and submissiveness in the other, play an important role in shared psychotic disorder.