This kind of anonymous letter is sent to someone in an effort to do him some harm and to be unkind, not as part of a greater crime such as kidnapping or ransom seeking. Unfortunately, letters of this kind are rarely followed up and the perpetrators are rarely prosecuted as the police do not make such cases a priority. However, such letters can breed atmospheres of distrust that are insupportable to the recipients. Thus, these letters must be recognized as criminal actions, representing a kind of liberation of some grudge that the anonym has against the addressee. What has been found to be true about these anonyms is that they are generally persons who are not openly disagreeable, who seem to attach importance to respectability (and thus will not come out and say openly to their victims what they do feel comfortable writing in a letter), and they often tend to be inconsequential people of little importance to the addressee. Their material or emotional life is not satisfying, and they may have recently been severely disappointed and disillusioned (not receiving an inheritance or approval or being rejected). The anonym probably lives in the same area as the addressee and will have some way of witnessing the victims' reactions to the distressing letter. The writer may be a member of the family, an unnoticed employee, a slighted friend, or a neighbor who has never been noticed by the victim. These letters seem to be a cry of the powerless from persons who have lost the ability to speak their mind and have been pushed to the limit of their understanding and patience. Because these letters lead to rashness, mistrust, false accusation or even violence and crime, their authors must be identified, and can be if these characteristics are taken into account. Several case examples are cited. --in French.