OBJECTIVE To clarify the natural history and frequency of thyroid echo abnormalities in a random adult population by performing a 5-year follow-up study of subjects of a previous thyroid ultrasonographic (US) screening study. METHODS In the original survey, 253 randomly selected adults were screened by means of thyroid US. US abnormalities were detected in 69 subjects (27%). In the follow-up study, 57 (83%) of those 69 subjects who had abnormalities were reexamined by means of thyroid US, fine-needle aspiration biopsy (FNAB), blood tests, and clinical examination. RESULTS Of 34 individual nodules, 12 (35%) had grown. Biopsy was performed in 10 of them. Nine were benign. One was equivocal, was excised, and proved to be an adenomatous nodule. Eight nodules (24%) had diminished or disappeared. Seven new focal lesions were found in seven subjects (12%). Biopsy was performed in five of these lesions, and they were benign. At 5-year follow-up, no thyroid malignancies were detected among subjects with echo abnormalities at the primary US screening. CONCLUSIONS Thyroid US abnormalities occurring in a random adult population are predominantly benign and clinically unimportant.