Poorly differentiated thyroid carcinoma (PDTC) is defined as a malignant follicular cell derived neoplasm, both morphologically and biologically intermediate between well differentiated and anaplastic thyroid carcinoma (ATC). In the present study we investigated the expression levels of two distinct sets of miRNAs (‘set 1’: miRNA-146b, −181b, −21, −221, −222, all shown to be significantly upregulated in papillary thyroid carcinoma [PTC]; ‘set 2’: miRNA−30d, −125b, −26a, −30a−5p, and let7c, all downregulated in ATC) in a series of 15 PDTC (including 3 mixed PDTC/PTC), 9 ‘pure’ PTC, and 9 ATC. Compared to normal thyroid tissue all ‘set 1’ miRNAs were significantly upregulated in PTC (p<0.001); in ATC 4/5 miRNAs were upregulated (p<0.001) whereas in PDTC the expression levels of all 5 miRNAs did not differ significantly from normal thyroid. All miRNAs of ‘set 2’ were significantly upregulated in PTC (p<0.004) and downregulated in ATC (p<0.03); in PDTC only 3/5 were downregulated (p<0.011). All 10 miRNAs investigated differed significantly (p<0.003) between PTC and PDTC. In the histologically differentiated PTC compound of mixed PDTC/PTC cases, however, miRNA expression levels of all 10 miRNAs investigated lacked significant difference from those found in the PDTC compound, whereas 6/10 miRNAs differed significantly from ‘pure’ PTC. Our results indicate that analysis of distinct sets of miRNAs represent useful tools to distinguish PDTC from ‘pure’ PTC. Additionally our findings suggest that lack of deregulation of some miRNAs may select a subset of PTC prone to progression to PDTC.