No single cytologic feature is specifically diagnostic for papillary thyroid carcinoma. We report herein the presence of swirl-like cellular aggregates in fine needle aspirates of papillary thyroid carcinoma but not in other thyroid entities. Cellular swirls are defined as concentrically organized aggregates of tumor cells in which many of the most peripherally situated cells have ovoid rather than round nuclei that are oriented perpendicular to the radius of the swirl. One hundred Papanicolaou- and/or Diff-Quik-stained FNAs of the thyroid diagnosed as papillary carcinoma, including seven fine needle aspirates of cervical lymph nodes showing metastatic papillary carcinoma, with or without cell blocks, were reviewed for the presence of cellular swirls. An additional 100 thyroid FNAs, similarly stained and prepared, diagnosed as nodular goiter, Hashimoto's thyroiditis and follicular neoplasm were also reviewed for the presence of cellular swirls. Cellular swirls were easily observed at screening magnification and confirmed at high magnification. Seventeen of 100 FNAs (17%) of papillary carcinoma contained cellular swirls. No cases diagnosed as nodular goiter, Hashimoto's thyroiditis or follicular neoplasm contained these structures. Thirteen cases with swirls had histologic follow-up. These comprised seven papillary carcinomas with classical histopathology, two designated 'differentiated papillary carcinoma,' two with follicular variant histopathology; one with a minor component of follicular variant histopathology; one papillary carcinoma metastatic to a cervical lymph node with classic histopathology. Swirls occurred in cases with relatively little pleomorphism, or in well-differentiated regions of papillary carcinoma that also displayed less well-differentiated components. Cellular swirls are a finding that is highly specific to papillary thyroid carcinoma. They are easily seen at screening magnification. Their presence in a FNA specimen may be helpful in cases where classic criteria for papillary thyroid carcinoma are scarce, particularly in well-differentiated papillary thyroid carcinoma. While the size and scope of this study are insufficient to conclude that cellular swirls alone are diagnostic of papillary thyroid carcinoma in the absence of other criteria, we believe these structures should be added to the list of diagnostic criteria.