Emphysematous psoas muscle abscess has rarely been described and has not been reported to be associated with ruptured mycotic aneurysm. We report two cases of ruptured mycotic iliac arterial aneurysm complicated by emphysematous abscess of the left psoas muscle. Case 1 occurred in a 70-year-old man and Case 2 in a 63-year-old woman. Both patients presented with fever for several weeks. Clinical clues leading to the diagnosis included a palpable abdominal mass with (Case 2) or without (Case 1) pulsation, blurring of the psoas muscle shadow with abnormal gas distribution on the plain abdominal film (Case 1), and peripheral vascular insufficiency and Salmonella bacteremia (Case 2). Ruptured mycotic aneurysm of the left iliac artery complicated with left psoas muscle abscess was clearly demonstrated by abdominal computerized tomography scan and intravenous digital subtraction angiography in both cases. Causative agents, multi-drug resistant Acinetobacter baumannii and Klebsiella pneumoniae, unusual pathogens for mycotic arterial aneurysm, were cultured from debrided tissue in Case 1, and this finding led to the speculation that the infection was hospital-acquired. The favorable outcome in Case 2 resulted from early vascular surgery and a prolonged course of effective antimicrobial therapy.