The purpose of this study was to evaluate the usefulness of an ultrasound-guided fine-needle aspiration biopsy (US-FNAB) for the evaluation and treatment planning of nonpalpable thyroid lesions, including infracentimetric nodules. One hundred and twenty one patients underwent US-FNAB for 149 non-palpable solid nodules. Sixty-five patients underwent surgery, and 84 were followed up for at least 36 months. The results of the US-FNAB correlated with the pathological findings and clinical follow-up results. The nodules ranged from 0.3 to 2 cm in diameter, with a mean of 0.8 cm. Among the 149 nodules, 115 were infracentimetric and 34 were centimetric or supracentimetric in size. Of the 149 thyroid nodules, US-FNAB was true positive in 43, true negative in 90, false positive in 7 and false negative in 1. In 8 cases, the lesion was inadequately sampled. The sensitivity, specificity, positive predictive value, negative predictive value and accuracy of the US-FNAB for the infracentimetric nodules were 96.9, 93.4, 86.1, 98.6 and 94.4%, and for the centimetric or supracentimetric nodules, were 100, 90.5, 85.7, 100 and 93.9%, respectively. There were no significant differences in the results between the infracentimetric and centimetric or supracentimetric nodules. In conclusion, an US-FNAB is a useful tool for determining the treatment plan of non-palpable solid thyroid nodules, even when less than 1 cm in diameter, and shows high sensitivity, specificity and accuracy.