Mycotic aneurysms, a quite rare syndrome, are classified into three groups: primary (the most frequent one), secondary to bacterial endocarditis, and secondary to direct inoculation (infections focus or trauma). Primary aneurysms are related to bacteremia with bacterial setting of an atheromatous lesion, most often. Clinical presentation is often misleading or delayed from infectious process. However, presence of clinical aneurysm, pain, and infectious syndrome should suggest diagnosis and lead to immediate antibiotic therapy and rapid surgical management, as rupture of the aneurysm is a frequent, serious and ineluctable complication. Importance of microbiologic search is to be emphasized as the diversity of the responsible microorganisms is very high; waiting for bacteriological results, a bactericidal double antibiotherapy should be instituted.