The phenomenon of shared delusions was found in 9 (8.4%) of 107 personally investigated patients suffering from delusions of infestation (88 females, 19 males). A greater number of females (ratio of females to males 3.5:1) "induced" others, whereas a gender ratio of 1:1 was evident in the group of affected patients. Since the ratio of blood relations to non-blood relations was 1:2.3, genetic factors seem to play a less important role than the direct impact of deluded patients on their environment. The rare occurrence of shared delusions of infestation leads to the conclusion that only persons with a paranoid predisposition (paranoide Bereitschaft) may be affected. Cases of induced delusions are also described in which attending physicians act as "inducers".