The diagnostic accuracy of fine-needle aspiration biopsy of the thyroid was evaluated. Between June 1982 and July 1987, 354 fine-needle aspiration biopsies were performed on 289 patients with thyroid nodules. The ages of the patients ranged from 21 to 86 years (median, 45 years); 61 (21%) were men and 228 (79%) were women. Surgical confirmation of the cytologic diagnosis was obtained in 59 patients. Fine-needle aspiration biopsy identified 10% of the nodules as probably malignant and 10% as possibly malignant. Of these nodules, 30.5% were proved to be malignant by histologic examination. The accuracy of distinction between a benign (class I or II) or probably malignant (class IV) diagnosis was 95.2%. Patient acceptance of this procedure was good and complications were infrequent. These results indicate that fine-needle aspiration biopsy of the thyroid is accurate, safe, well tolerated, and easily applied, without its highly desirable features being compromised, even when a moderate number of such procedures are performed.