Until the second half of the nineteenth century the word paranoia was used to mean 'out of mind”. Kraeplin has been determinant to change the conceptual history of paranoia, leading, with the contribution of other authors, to the emergence of delusional disorder in current classifications.

We intend to characterize delusional disorder and its clinical utility.

We describe and analyse a case of a man with the diagnosis of delusional disorder – persecutory subtype and make a bibliographic review about this disorder.

The delusional disorder has a prevalence of 0,03%. The key feature is the existence of an encapsulated delusional system without impairment of other mental functions. As expected our patient has personality traits characterized by suspiciousness, rigidity, an inappropriate sense of personal rights and a tendency to misconstrue the actions of others as hostile. His defense mechanisms are mainly denial, projection and reaction formation. The beginning of the disease was between 35-45 years. First time he was observed by Psychiatry he was taken by family and recently it was by court order. The differential diagnoses were excluded (paranoid personality, schizophrenia, dementia, other medical conditions, disorders induced by substances).

The cause of delusional disorder is unknown. The importance of personality traits, life events and genetic factors seems to be uncertain and probably variable. The typical lack of insight conditions the therapeutic adhesion and establishing the necessary adequate therapeutic relationship remain a big challenge.