Fine-needle aspiration is a low-cost diagnostic tool with principal value in determining which patients with thyroid nodules should undergo surgery. Team work and close cooperation among endocrinologists, surgeons, and pathologists are essential for success. Cytologic criteria for diagnosis of the most frequent conditions (benign cystic lesions), Hashimoto thyroiditis and malignancies found in thyroid aspirates have been provided. The unsolved problem of the so-called "follicular" or oxyphilic lesion or neoplasia will be investigated by immunocytochemistry.