PURPOSE: The aim of this study is to determine the efficacy of ultrasound-guided fine-needle aspiration biopsy (US-FNAB) of thyroid nodules that are equal to or less than 1 cm at the maximum diameter. MATERIALS AND METHODS: The US-FNABs performed on thyroid nodules from March to August 2009 were included in this study. We retrospectively evaluated the cytopathologic results as well as any complications associated with the procedure. RESULTS: Of the 241 thyroid nodules (range: 0.1 - 1.0 cm, mean size: 5.8 mm) in 184 patients (female: male = 164:20, mean age: 49.0 years, age range: 18 - 77 years old), the incidence of an inadequate sample was 14.1% (34/241) for the US-FNABs. Eighty six nodules were surgically removed in 62 patients, of which 15 were confirmed to be benign nodules and 71 were confirmed to be malignant nodules. The number of true positive, false positive, true negative and false negative results for US-FNAB were 59, 0, 10 and 3%, respectively, and the sensitivity, specificity, positive predictive value, negative predictive value and accuracy were 95.2%, 100%, 100%, 76.9% and 95.8%, respectively. All the false negative cases were less than 3 mm at the maximal diameter. There were no serious complications in all the patients. CONCLUSION: US-FNAB was effective for the cases of thyroid nodule under 1 cm at the maximal diameter. However, a false negative result of US-FNAB should be considered for the cases of very small nodules that are less than 3 mm at the maximal diameter.