Cytologic features of poorly differentiated 'insular' carcinoma of the thyroid, as revealed by fine-needle aspiration biopsy. The authors report on the cytologic features of six cases of poorly differentiated "insular" carcinoma (IC) of the thyroid, a recently described variety of thyroid tumor intermediate between well-differentiated and anaplastic neoplasms. It is characterized by trabecular and/or alveolar growth patterns, merging with follicular areas, and by the absence of pleomorphism in the tumor cells. Fine-needle aspiration biopsy (FNAB) materials (both smears and cell-block preparations) from six patients were reviewed after the diagnosis of IC was confirmed on the surgical specimen. The following cytologic features were consistently found: high cellularity and necrotic background; low grade of atypia; trabeculae and/or clusters, possibly associated with microfollicles, of cells with poorly defined cytoplasm; and cytoplasmic vacuoles containing thyroglobulin. Nuclear inclusions and grooving of the nuclear surface were additional features. Preoperative diagnosis suggestive of IC might be of value for planning surgical treatment and subsequent therapy. 