BACKGROUND: Tumor-associated macrophages (TAMs) play a tumorigenic role related to advanced staging and poor prognosis in many human cancers including thyroid cancers. Yet, a functional role of TAMs in papillary thyroid carcinoma (PTC) has not been established. The aim of this study was to investigate TAM expression in human PTC with lymph node (LN) metastasis. METHODS: Thirty-six patients who underwent surgery after being diagnosed with PTC with LN metastasis were included. Primary tumor tissues were immunohistochemically stained with an anti-CD68 antibody and clinical characteristics according to TAM density were evaluated. RESULTS: The TAM densities (CD68+ cells) varied from 5% to 70%, in all tumor areas, while few cells were stained in adjacent normal tissues. TAMs were identified as CD68+ cells with thin, elongated cytoplasmic extensions that formed a canopy structure over tumor cells. Comparing clinicopathologic characteristics between tumors with low (<25%) and high (25% to 70%) TAM densities, primary tumors were larger in the high density group than in the low density group (2.0+/-0.1 vs. 1.5+/-0.1; P=0.009). CONCLUSION: TAMs were identified in primary PTC tumors with LN metastasis and higher TAM densities were related to larger tumor sizes, suggesting a tumorigenic role of TAMs in human PTCs.