Thyroid nodules are the most common thyroid diseases. The discovery rate of thyroid incidentaloma has been increasing as a consequence of the widespread use of thyroid ultrasound examination (US), but the identification and management of thyroid malignancy are not well estabilished. To determine the prevalence and the risk factors of malignancy in patients presenting with thyroid incidentalomas, the authors carried out retroprospective studies on the findings of US and the cytological results. Five hundred and six consecutive patients that had visited Endocrinologic Clinic of Chonnam University Hospital with thyroid incidentalomas were included in this study between January 2000 and May 2003. The nodules were diagnosed by US using a 10-MHz transducer, and then US-guided fine needle aspirations (FNA) were performed. We retrospectively analyzed the characteristics of sonographic features and cytological results. The series included 441 females and 65 males, with a mean age of 48.1 12.0 yr (ranging from 16 to 80 yrs). Thyroid malignancies were observed in 46 of 506 (9.1%) thyroid incidentalomas. Age and sex were of no value in determining the risk of malignancy. Logistic regression indicated that a hypoechoic feature (p=0.019) and the presence of calcification (p<0.001) were useful criteria in predicting malignancy. The prevalence of malignancy was significantly higher in nodules with hypoechoic appearance or calcification. None of the other US parameters, such as characteristics of nodular margin, the presence of halo, the presence of cervical lymphadenopathy, nodule number, nodule size, significantly associated with the risk of malignancy. The prevalence of malignancy in nonpapable thyroid incidentaloma was 9.1%, similar to that of palpable nodules in previous studies. It is advised that nonpalpable thyroid incidentaloma should no further examined with US-guided FNA to distinguish thyroid nodules that might have a higher risk of malignancy. US findings are useful in selecting higher risk group in nonpalpable lesions.