Fine-needle aspiration biopsy (FNAB) is important for classifying thyroid lesions. Thin-layer cytology (TLC) has been adopted in thyroid cytology with encouraging results and its efficacy in diagnosing nodular lesions in a 1-year period was evaluated. All 2006 thyroid FNAB processed by TLC only in the year 2004 were examined. The 2002 and 2003 series, processed both by TLC and conventional smears (CS), were used as controls. All FNAB were classified according to previously published morphologic criteria. Surgery was performed on 311 cases: all 98 benign and 30 malignant lesions were histologically confirmed. The inadequate diagnoses resulted 11.3% in 2004, 13.3% in 2003, and 18.2% in 2002. The indeterminate diagnosis rate was 16.9% in 2004, 20.7% in 2003, and 24.8% in 2002. Sensitivity, specificity, and diagnostic accuracy values ranged from 80% to 100%. TLC is useful in reducing inadequate and indeterminate cases without decreasing the preoperative diagnostic accuracy. Ancillary techniques can be successfully applied to the stored material.