To determine the diagnostic pitfalls of thyroid fine-needle aspiration (FNA), we reviewed 394 thyroid aspirates obtained between January 1986 and December 1990. Surgical follow-up was available for 150 aspirations. The cytologic diagnoses were categorized into four groups: benign, 57; indeterminate, 51; malignant, 33; and nondiagnostic specimen, nine. There were three false negative diagnoses (3%), which upon review were judged to be inadequate specimens. Three false positive diagnoses (7%) were identified: in the first two cases, follicular adenomas were mistaken for papillary carcinoma; in the third case, atypical Hürthle cells were mistaken for a Hürthle cell carcinoma. Our results showed a sensitivity of 93% and a specificity of 91% for the detection of malignancy. If indeterminate cases were considered positive, the specificity decreased to 50%, while the sensitivity increased to 97%. We conclude that: 1) certain follicular adenomas may display cytologic features mimicking papillary carcinoma; 2) as in follicular neoplasms, aspirates of Hürthle cell adenomas cannot be differentiated from Hürthle cell carcinomas; 3) with adequate sampling, false negative results can be markedly reduced.