Four pure insular carcinomas (IC) and one IC with focal anaplastic carcinoma (AC) of the thyroid with cytologic evaluation by fine-needle aspiration (FNA) were reviewed. The needle aspirates from the four pure ICs revealed abundant monomorphic follicular cells present singly, in small, loose aggregates, and in cohesive trabecular and acinar clusters. Tumor cells showed fragile, ill-defined, granular cytoplasm and oval nuclei with conspicuous or inconspicuous nucleoli. The case of IC with focal AC yielded, in addition to the follicular cells as seen in the FNA of the 4 cases of pure IC, large pleomorphic malignant cells with prominent nucleoli that were characteristic for an AC, giant-cell type. No intact insulae of tumor cells were identified in any of the 5 cases. Thus, a thyroid IC may be suspected if abundant cohesive and dyshesive monomorphic follicular cells are present in the tumor FNA. However, a firm diagnosis of thyroid IC can only be made by histologic examination of the excised tumor. Diagn. Cytopathol. 25:325–330, 2001. © 2001 Wiley-Liss, Inc.