A family is presented in which both parents and all ten children possessed the same persecutory delusions. The etiological importance of noxious environmental stimuli is stressed. A developmental scheme for folie à deux is suggested: a phase of sensory isolation and ideational deprivation which is disorganizing and regressive, followed by a phase of identification with the aggressor (the psychotic parent), and a re-indoctrination period when delusional ideas are incorporated. The author feels that the process is similar to that which occurs in brainwashing, hypnosis, and psychotherapy.