Several studies have shown that ultrasound guidance can serve as a valuable aid in improving the diagnostic yield of fine-needle aspiration (FNA) biopsy of thyroid nodules. In this study, we evaluated the combined impact of ultrasound-guidance, rapid on-site evaluation of FNA specimens, and different cytologic preparations (fresh and alcohol-fixed smears, Millipore filter) and staining methods (Diff-Quik and Papanicolaou stains) on the diagnostic yield of thyroid FNA. Ultrasound-guided FNA was performed on 282 patients (313 cases) between November 1997 and April 1999. The diagnostic categories included: benign (198 cases, 63.2%); indeterminate (42 cases, 13.4%); suspicious for follicular variant of papillary carcinoma (26 cases, 8.3%), malignant (32 cases, 10.1%); and nondiagnostic (15 cases, 5%). The nondiagnostic cases also included 6 cystic lesions without any solid component and 3 thyroid-bed aspirations. After excluding these, the nondiagnostic rate was only 2%. Histological follow-up was available in 77 (77/313) cases. The concordance rate between cytological and histological diagnosis was 100% in malignant, 67% in suspicious, and 56% in indeterminate cases. All cases with histologic follow-up were selected to evaluate the independent diagnostic efficacy of each aforementioned cytologic staining method. A definite diagnosis could be made solely on the basis of air-dried, Diff-Quik-stained preparations in 50 (65%), alcohol-fixed, Papanicolaou stained smears in 68 (88%), and Millipore filter preparations in 70 (91%) cases. We conclude that ultrasound-guided FNA combined with on-site evaluation and different cytologic preparations can significantly improve the diagnostic accuracy of thyroid FNA specimens. Diagn. Cytopathol. 2000;23:425–429. © 2000 Wiley-Liss, Inc.