In one case, the teenage girl lied about incest with her stepfather because she resented her mother's remarriage. In another, the child's need to lie to avoid disclosing her masturbation practices was abetted by her mother's delusional conviction that her new husband was sexually betraying her. Another child misinterpreted an actual incident out of a desire for revenge and because of sickness. Other cases illustrate maternal delusion that incest was taking place between the husband and daughter. Expressed fantasies about incest can be as devastating to a family as actual incest. Moreover, failure to recognize a maternal delusion of incest may place the child at risk for physical abuse by the psychotic mother. Another case illustrates a false denial of incest that was brought about by the mother's insistence. Untrained interviewers often accept false denials of incest at face value, which may lead underestimation of the incidence of incest and to the assumption that many accusations are false. Investigators should be aware of their own biases toward teenagers or overly dramatic women. They should take the position that they do not yet know what really happened in order to allow the family latitude to talk. Investigators should see each family member individually, maintaining open lines of communication and insisting upon clarity. Each parent should give a detailed sexual, psychosocial, and family history. The investigation should not be traumatic for the family. The investigator must keep in mind that actual paternal incest is common in psychiatric patients and that incest delusions and hoaxes are rare. Other guidelines are suggested. Five case examples are included.