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Injection drug users frequently present with abscess, cellulitis, and endocarditis. The development of arterial mycotic pseudoaneurysm (AMP) as a sequela of injection drug use, however, is much less frequently reported. We undertook a study to determine the prevalence and presenting clinical characteristics of AMP, utilizing a retrospective review of all emergency department cases seen at one city public hospital for the 5-year period 1994-1999. Initial evaluation included physical examination, CT scan, ultrasound, and/or angiography. There were 7,795 patient visits for complications of injection drug use; 11 patients had AMP (0.14%). AMP involved the brachial (n = 5), subclavian (n = 2), radial (n = 2), femoral (n = 1) and popliteal arteries (n = 1). Fever was absent in the majority of patients (7/11). Either pulsatility or a mass was noted in three cases, and both were seen in 6/11 (54%). AMP was not initially suspected in three cases, which were treated as abscesses and surgically incised, resulting in arterial rupture. The annual prevalence of AMP in the presenting population was estimated to be 0.03%. However, a high index of suspicion for AMP should be maintained with injection drug users presenting with a mass or pulsatility over an artery, as there is risk of rupture, rapid exsanguination, and distal embolization.