We evaluated the clinicopathologic, immunohistochemical, lectin histochemical, ultrastructural and morphometric characteristics of a series of 28 poorly differentiated carcinomas of the thyroid (PDCT). The 28 tumors were classified according to their predominant growth pattern as insular (n=12), trabecular (n=10) and solid subtypes (n=6). The overall mortality rate was 46%. No significant differences were found among the 3 subtypes. Data obtained by lectin histochemistry, electron microscopy, and morphometry point to a closer relationship of PDCT to follicular than to papillary carcinoma. Our results do not support the subdivision of PDCT into subtypes.