The concept that poorly differentiated carcinomas (PDC) represent a group in an intermediate position in the spectrum of follicular cell-derived carcinomas of the thyroid gland is currently well established. Because at the well-differentiated end of the spectrum there are two groups of entities with distinct biological characteristics, ie, the papillary carcinoma (PC) and the follicular carcinoma (FC), we examined the group represented by PCs to ascertain whether papillary carcinoma-related PDCs (pPDC) represent merely a histologic variant or a distinct pathologic entity. For this purpose 227 consecutive PCs were reclassified according to current criteria. The association between the presence of a tumoral pattern consistent with pPDC (response variable), and prognostic factors such as gender age, pTNM (predictive variables) was evaluated in terms of odds ratio statistics. One hundred eighty-three of 227 cases, defined as PCs met the World Health Organization criteria of the not otherwise specified (NOS) (79 cases), microcarcinoma (65 cases), encapsulated (4 cases) and follicular (35 cases) variants. Forty-four cases, defined as pPDCs, met those of the tall cell (39 cases), columnar cell PC (2 cases) and mixed tall cell-columnar cell PC (3 cases) variants. Statistical analyses of the two groups of patients showed a significant correlation between differentiation and age above 40 years, extrathyroid tumor extension and low ratio of regional nodal involvement at the onset of disease. More strikingly they also showed that morphology, ie, a tumoral pattern consistent with pPDC (differentiation), is the strongest predictor of biological behavior including recurrences and recurrence-related deaths that appear to occur five and twenty times more frequently in pPDCs than in PCs, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)