Objective: This paper is written with the aim of presenting a series of cases of folie à deux, an unusual disorder, occurring in a forensic setting where all the victims suffered a fatal or near-fatal outcome. Similarities in the psychopathology in these cases are drawn and comments are made about their outcome in court. The paper is also written to draw attention to the potential risks involved in this type of case and to accent patterns in psychopathology and in legal outcomes. Clinical picture: Five folie à dew cases are described. All are well known to one of the authors (NK), who has treated them at some stage of their illness, provided medico legal reports for the trial of cases 3, 4 and 5, and provided expert witness testimony in the trial of case 3. All case notes and other relevant documents were studied in detail and a brief summary is provided for each case. Outcome: The cases appear to conform with the description of folie a deux given in the literature and consist of husband and wife (three cases), mother and daughter (one case) and twin brothers (one case). None of these cases had had any past psychiatric or criminal history, yet all the victims suffered a fatal or near fatal outcome. Additionally, all had shared religious delusions in one form or another. Conclusions: Folie a deux cases with shared religious delusions can be a very high risk and could be potentially fatal in a family setting. The authors also highlight the difficulties inherent in a forensic system restricted to McNaughton rules, particularly when a person is on trial having committed, or being alleged to have committed, a serious offence, and who is found to be suffering from a psychotic illness.