['Signal transducer and activators of transcription-3 (STAT3) plays a critical role in promoting survival and cell growth as well as facilitating angiogenesis and metastasis in several cancers.', 'This investigation focused on evaluation of STAT3 activities in human papillary thyroid cancers (PTC).', 'STAT3 activities of nuclear extracts of tumor tissue were measured from 35 PTC patients using enzyme- linked immunosorbent assay-based kits.', 'STAT3 activities of PTC tissues were significantly lower than those of surrounding normal thyroid tissues [0.36 (interquartile range 0.24-0.72) vs 0.50 (0.29-1.11) arbitrary units, p<0.01]. We further analyzed the association between STAT3 activity and clinicopathologic factors in PTC tissue. Tumors with size ≥2 cm displayed significantly lower STAT3 activities than those <2 cm [0.25 (0.21-0.37) vs 0.53 (0.37-0.61) arbitrary units, p<0.01]. Notably, tumor size was inversely correlated with STAT3 activities in T1799A BRAF mutation-positive cases (Rs=-0.58, p<0.05), but not mutation-negative cases.'] Are basal STAT3 activities negatively correlated with tumor size in papillary thyroid carcinomas?