Poorly differentiated carcinoma is adopted as an independent entity in World Health Organization (WHO) and in General Rules for the Description of Thyroid Cancer by The Japanese Society of Thyroid Surgery (JSTS), but their definitions are significantly different. In this study, we investigated the prevalence and clinical significance of poorly differentiated carcinoma by WHO classification (poorly differentiated carcinoma [WHO]) and that in JSTS (poorly differentiated carcinoma [Sakamoto]), and tall cell variant in papillary carcinoma. In our series of 1,707 patients, 186 (10.9%), 15 (0.9%) and 62 (3.6%) were diagnosed as poorly differentiated carcinoma (Sakamoto), poorly differentiated carcinoma (WHO) and tall cell variant, respectively. Poorly differentiated carcinoma (WHO) and tall cell variant independently affected cause-specific survival (CSS) of patients, but poor differentiation (Sakamoto) did not. Therefore, it is suggested that poorly differentiated carcinoma (Sakamoto) predicted the likeliness of carcinoma recurrence, it is inappropriate to separate it as an independent histology.