Introduction: Clinically unsuspected pulmonary embolism (PE) can be detected in oncology patients undergoing computed tomography (CT) imaging for reasons other than for PE diagnosis, but there is little prospective data on its true prevalence, clinical importance, or on methods to improve detection.Methods: In consecutive oncology patients undergoing CT imaging of the chest for indications other than PE detection, CT pulmonary angiography (CTPA) was systematically included as part of the imaging protocol. Each imaging study was prospectively analyzed for the presence of PE. A 6-month follow-up was performed. Institutional review board approval was obtained.Results: Four hundred seven oncology patients were included. Indications for chest CT imaging included baseline staging (31%), restaging after therapy (53%), routine surveillance (15%), or assessment of extrathoracic disease (1%). Clinically unsuspected PE were detected in 18 patients (4.4%). The prevalence of unsuspected PE was 6.4% among inpatients and 3.4% among outpatients. PE was more prevalent among patients with metastatic disease (7% versus 2%, p = 0.007) and in patients who had received recent chemotherapy (11% versus 3%, p = 0.008). In 7 (39%) of the 18 patients with clinically unsuspected PE, emboli were only identifiable on the CTPA study and not on the routine chest CT study. The diagnosis of PE led to immediate changes in patient management.Conclusion: Clinically unsuspected PE is present in up to 4.4% of oncology patients undergoing CT imaging for indications other than PE diagnosis. Modifying standard CT imaging protocols to include a CTPA examination optimizes their detection and leads to changes in patient management.