Early diagnosis of a mycotic aneurysm is critical, but often unsuspected, due to the insidious onset of symptoms related to occult infection. This study was undertaken to assess the role of leukocyte scintigraphy in establishing the diagnosis of mycotic aneurysm. The records of all patients with possible mycotic aneurysm between 1985 and 1991 were reviewed. Seven patients had leukocyte scintigraphy and computed tomography (CT), three also had magnetic resonance imaging (MRI) and three had angiography as part of the diagnostic workup. CT and MRI detected aneurysms in five of the seven patients, but CT scans were misinterpreted in two patients as indicative of abscess only. In six patients, infection could not be differentiated from thrombosis, seroma or hemorrhage by CT or MRI. Leukocyte scintigraphy was positive in all four patients with infected aneurysms; it was negative in two of the three noninfected aneurysms and equivocal in the third. Leukocyte scintigraphy provided a useful early survey that demonstrated evidence of infected aneurysms in four patients and identified other sites of infection in two patients. Leukocyte uptake complemented CT, MRI and angiographic findings distinguishing between seroma/hematoma and adjacent infection to establish a preoperative diagnosis of infected aneurysms.