Fine-needle aspiration (FNA) cytology is widely used to examine thyroid lesions. However, its diagnostic accuracy is limited by the narrow choice of cytopathologic markers indicative of invasive/metastatic powers of a tumor. The aim of this study was to identify features that may serve as such indicators. We have examined FNA smears of 50 histologically proven papillary thyroid carcinoma (PTC) cases applying computer-assisted morphometry to assess patterns formed by PTC cells. Cytokeratine (CK) 8 immunocytochemistry was used to verify the epithelial origin of cells under study. All analyzed smears contained blood, histiocyte-like cells and CK8-positive follicular cells occurring both as single cells and in monolayer cell sheets. In 60% of cases we revealed cell sheets displaying two distinct cell patterns. The first one (pattern R) consisted of moderately pleomorphic, rather regularly arranged cells having an amphophilic cytoplasm. The second one (pattern I) was formed by highly pleomorphic cells with a basophilic cytoplasm. Patterns R and I were clearly different in cell size and shape as well as in nuclear size and shape. These patterns were never observed within the same cell sheet indicating that they may be formed by different subclones of tumor cells. Thus, it can be concluded that PTC frequently displays two definitely different cell patterns. We think that these patterns have a potential to serve as indicators for early events of an invasive/metastatic process. It remains to be seen whether the simultaneous occurrence of these patterns is a PTC-specific feature. Diagn. Cytopathol. 2012;40:E55–E61. © 2011 Wiley Periodicals, Inc.