Twelve cases of mycotic aneurysm are reported. These rare lesions (2 to 3% of all aneurysms) may affect all arteries. Primary aneurysm is currently the most frequent type observed in France, with onset being related to a bacteremia or septicemia with development on an atheromatous lesion (8 of the 12 cases in this series). Manifestations are often misleading or diagnosis delayed in relation to the infectious process, apart from in rupture, which is the commonest cause of detection of aneurysm (half of the present series). Ideal surgical treatment is exclusion with extra-anatomic revascularization: but revascularization in situ has been reported by many authors (6 of the 12 cases in this series). Localization of aneurysm determines surgical procedure and also very variable mortality rate: 4 of the 12 patients in this series.