Fine-needle aspiration of the thyroid has been accepted as one of the initial diagnostic tools in the evaluation of thyroid nodules. As its use becomes more widespread, the demand for more precise diagnosis has increased. The histopathology of insular carcinoma of the thyroid is now well recognized. However, the cytologic diagnostic criteria are not well established. The reported series have been small (4–6 cases), which is not surprising because of the rarity of this tumor. They consist of retrospective reviews of the aspirates (after the histologic diagnosis had been made from the thyroidectomy specimens). Also, the case reports do not provide uniform cytologic criteria; this could be due to limited sampling of these tumors (which are usually large). A cytologic diagnosis of insular carcinoma can be suggested if multiple samples of a thyroidal mass are markedly cellular, with a cytologic pattern reminiscent of a follicular variant of papillary carcinoma. However, the follicular cells are arranged predominantly in rosettes, their nuclei appear more monotonous, some “intranuclear cytoplasmic pseudoinclusions” are seen, and there is an occasional large cell with a pleomorphic nucleus. Diagn. Cytopathol. 2006;34:572–575. © 2006 Wiley-Liss, Inc.