Smears of fine-needle aspiration (FNA) biopsy specimens of 24 histologically proven and MIB-1-positive hyalinizing trabecular adenomas stained with Papanicolaou or Diff-Quik were stripped from the original slides, transferred to charged slides, destained, and restained with MIB-1. Positive immunoreactivity with MIB-1 in a peripheral cytoplasmic pattern was present in 17 specimens, no immunoreactivity was seen in 4 specimens, and equivocal staining was seen in 3 specimens. Papanicolaou-stained smears of papillary thyroid carcinoma, medullary thyroid carcinoma, thyroid follicular neoplasm, and Hürthle cell adenoma (5 specimens each) were treated similarly. None of the control cases stained with MIB-1 had the peripheral cytoplasmic pattern. Hyalinizing trabecular adenoma can be distinguished reliably from other thyroid neoplasms by MIB-1 staining of destained cells from FNA biopsy specimens.