A case is presented, in which the author elaborates the hypothesis of a parasomnia with (incestuous) sexual activity. As such a type of parasomnia is not officially recognized in literature of sleep medicine, it is difficult and controversial to defend this hypothesis. As a consequence, there is a great reluctancy in medical, c.q. psychiatric circles, as well as in jurisprudence. The case even illustrates a possible defensive reaction, e.g. denial. Therefore, chances for science - and especially also for the patient - can get lost. The author asks his colleagues to give special attention to possible similar cases. Further studies are needed to give a scientific base for the existence of a 'parasomnia with sexual activity'.