A critical review of the literature reveals that different types of thyroid carcinomas have been interpreted as poorly differentiated, including the tall and columnar cell variants as well as carcinomas with insular, trabecular, and solid growth patterns. In some publications the growth patterns have been emphasized, while in others histologic patterns and cytologic features are considered important for identification. However, insular, trabecular, nodular, and solid growth patterns lack specificity because they can be observed in hyperplastic lesions and benign thyroid tumors. The cytologic features of the vast majority of poorly differentiated thyroid carcinomas are similar to or overlap with those of papillary or follicular carcinomas. Cytologic atypia, mitotic activity, and necrosis--believed by some investigators to be useful clues in the diagnosis of poorly differentiated carcinomas--do not reflect cell differentiation, especially in endocrine organs. It is therefore not surprising that the immunohistochemical profile and the molecular abnormalities described in this heterogeneous group of carcinomas lack specificity and are not useful diagnostic tools. Because poorly differentiated thyroid carcinomas have not been well defined, currently it is not possible to set the threshold between well-differentiated and poorly differentiated thyroid carcinomas with follicular phenotype. The authors believe that the vast majority of poorly differentiated thyroid carcinomas are in fact examples of papillary or follicular carcinomas with unusual growth patterns.