Purpose: To retrospectively determine the prevalence and natural history of incidental pulmonary emboli in oncology patients, the number of such cases reported at initial thoracic computed tomographic (CT) image interpretation, and the factors that contribute to underdiagnosis. Materials and Methods: Institutional review board approval, which included HIPAA-compliant access to protected health information and waived patient consent requirements, was obtained for this retrospective study. Four hundred three consecutive oncology patients (199 male, 204 female; age range, 14-87 years; mean age, 55 years) in whom adequate-quality multidetector thoracic CT was performed within a 10-day period for indications other than pulmonary emboli assessment were identified. There were 31 (7.7%) inpatients at the time of imaging. Each imaging case was reviewed by two independent radiologists, and all pulmonary emboli were confirmed by a panel of three thoracic radiologists. Clinical charts were reviewed for demographic data, embolus detection, and outcomes up to 2 years after the initial examination. Patient groups were compared by using χ2 and one-sided binomial tests. Sixteen (4.0%) of the 403 patients had pulmonary emboli. The highest prevalences were in patients with gynecologic malignancies (two of 13, 15%) and in those with melanoma (four of 41, 10%). Four (25%) of the 16 patients with emboli were identified at initial clinical CT image interpretation, and all had multiple emboli involving at least the lobar arteries. Missed emboli typically were solitary and involved smaller arteries; no other confounding factors were identified. Six (60%) of 10 patients with emboli who underwent any lower extremity imaging had deep vein thrombosis. With the exception of one patient, who was transferred back to the referring physician and lost to follow-up, all patients with reported pulmonary emboli were treated. Two patients had subsequent embolic events: one death despite treatment and one recurrent embolus in a nontreated patient. Incidental pulmonary emboli were seen in 16 (4%) oncology patients but were initially reported in only four of them. The small size of involved arteries contributes to the failed detection at initial CT image interpretation, and patients with emboli in these small vessels may have deep vein thrombosis or recurrent emboli.