PURPOSE: To evaluate the clinical implications of subsequent ultrasound (US) for incidental thyroid lesions (ITLs) detected on positron emission tomography/computed tomography (PET/CT) in patients with non-thyroidal cancer. MATERIALS AND METHODS: We retrospectively searched for ITLs in 2703 patients with non-thyroidal cancer on 3226 PET/CT examinations. We divided the patients into two groups: US-performed and US-not performed groups. Diagnostic performance of US in detecting malignancy was analyzed according to the PET/CT findings. RESULTS: The incidence of ITLs on PET/CT in patients with non-thyroidal cancer was 8.4% (226/2703). Death rate from disease and stage were not significantly different between the groups. The malignant rate of ITLs that had cytopathology was 39% (22/57). All malignancies were primary thyroid cancers except for one metastasis. Overall diagnostic performance of US was 87% sensitivity, 66% specificity, 66% positive predictive value (PPV), and 89% negative predictive value (NPV). The diagnostic performance of US according to the PET/CT findings was probable benign on PET/CT (n = 9): 100% specificity, 100% NPV; suspicious malignant on PET/CT (n = 22): 75% sensitivity, 90% specificity, 90% PPV, 75% NPV; and undetermined on PET/CT (n = 26): 100% sensitivity, 94% specificity, 91% PPV, 100% NPV. CONCLUSION: ITLs did not affect the prognosis of patients with non-thyroidal cancer whether subsequent US was performed or not. Subsequent US may be unnecessary for ITLs with probable benign findings on PET/CT.