Induced delusional disorder (or shared paranoid disorder), also known as folie à deux, is a fairly uncommon disturbance characterized by the presence of similar psychotic symptoms in two or more individuals. Most often the symptoms are delusional. Usually the 'primary' case, i.e. the individual who first develops psychotic symptoms, can be distinguished from one or more 'secondary' cases, in whom the symptoms are induced. We discuss the concept of shared paranoid disorder and consider various aetiological, clinical and diagnostic issues related to the disturbance. We also describe a case of folie à famille, this condition being a type of shared paranoid disorder. The case involves a couple and their 12-year-old son. The boy's father is the 'primary' case, whilst the boy and his mother are both 'secondary' cases. The boy was admitted to our child and adolescent psychiatric unit for in-patient treatment. Treatment success was moderate in terms of improving the features of folie à famille in the three individuals involved.