[Induction of obsessions--folie a deux].

The disorder folie à deux is described from the literature with the classical subdivision into four groups, and the characteristic features and involved mechanisms essential to the disorder are presented. Three cases are described, two of them concerning twins, namely a sister-sister and a brother-brother relationship, together with a mother-daughter relationship. The cases show the characteristics in folie à deux: 1. An intimate relationship; 2. Social isolation; 3. Delusions similar in content. Ways of treatment are described.