BACKGROUND: The purpose of this study was to assess the prevalence of thyroid nodules in healthy adults without a history of thyroid disease and the results of fine needle aspiration cytology (FNAC). METHODS: We retrospectively studied 4,832 adults (2,427 women, 2,405 men) over the age of 20 who had visited our health care center from January, 2005, to March, 2008. Subjects with previous thyroid disease were excluded. All were screened by thyroid ultrasonography and FNAC was performed on large or potentially malignant nodules. RESULTS: Thyroid nodules were present in 686 women (28.3%) and 396 men (16.5%), with a female predominance (odds ratio = 1.47, 95% CI = 1.35~1.60). The prevalence of a thyroid nodule was significantly correlated with age in both women and men (P < 0.001). Multinodularity also increased according to age in both groups. Ninety patients were tested with conventional FNAC and 195 underwent ultrasonography-guided FNAC. The rate of inadequate cytology by ultrasonographic guidance was lower than by freehand methods, and the total rate of malignant cytology per patient was 17.9%. Ultrasonographic characteristics that significantly correlated with histologically-confirmed papillary carcinoma included a solid component, hypoechogenecity, irregular margin, and the presence of microcalcification or macrocalcification. CONCLUSION: The prevalence of thyroid nodules detected by ultrasonography was 28.3% in healthy women and 16.5% in healthy men population. The prevalence and multinodularity was significantly correlated with age in both groups. It's useful to examine thyroid by ultrasonography because of detecting more nodules, providing guidance of FNAC, achieving more adequate sampling and not missing small malignant nodules.