The fate of benign thyroid nodules has been unknown because there has been no study in this regard. We re-examined 134 patients with thyroid nodules who had had benign aspiration biopsy cytology 9 to 11 years ago. The thyroid gland was palpated by the same two thyroidologists throughout the study. Ultrasonography, fine-needle aspiration biopsy (FNAB), and ultrasound-guided FNAB were employed to examine the nature of nodules of 9 to 11 years' duration. Patients (n = 61) who had nodules difficult to palpate (small nodules), multiple nodules, or cystic nodules with papillomatous proliferation underwent ultrasound-guided FNAB; patients (n = 55) having a distinctly palpable single nodule underwent usual FNAB. None of the patients received any medical or surgical treatment. There were 86 single nodules, 14 multiple nodules, and 34 cystic nodules on the first examination. These benign nodules were reexamined for changes in size and cytology 9 to 11 years later. The most striking finding was a decrease in size or disappearance of the nodule in 42% to 79% of benign nodules. About 92% of nodules remained benign without changing cytologic classification. Only one case (0.9%) previously regarded as benign turned out to be malignant; this nodule grew in size compared with the previous examination. Among single and multiple nodules, 21% to 23% of the nodules increased in size; however, most patients with enlarged nodules (86%) showed the same class 2 cytology as before. Our present study indicates that biopsy-proved benign thyroid nodules remain benign over a prolonged period. Thus no medical or surgical treatment is required so long as the nodules do not grow.