PURPOSE: To evaluate the usefulness of ultrasonography in the differentiation between benignity and malignancy in the evaluation of small thyroid nodules (the longest diameter <10 mm). MATERIALS AND METHODS: Fine needle aspiration or surgery was performed for the evaluation of seventy two (44 benign and 28 malignant) small thyroid nodules from fifty eight patients. Ultrasonographic findings were retrospectively reviewed for the margin, echogenecity, presence of microcalcification, intranodular cystic change and peripheral hypoechoic halo. RESULTS: Ill-defined margin was more common in malignant nodules (61% vs 11%, p<0.05). In internal echogenecity, either iso- or hyperechogenicity was more coomon in benign nodules (45% vs 18%, p<0.05) while malignant nodules were often markedly hypoechoic (39% vs 18%, p<0.05). Malignant nodules contained more microcalcifications (36% vs 9%, p<0.05) whereas benign nodules often showed intranodular cystic change(39% vs 7%, p<0.05) and hypoechoic halo (41% vs 18%, p<0.05). CONCLUSION: Ultrasonography can be useful in the differentiation between benign and malignant small thyroid nodules.