Abstract Introduction: Tumor associated macrophages (TAMs) play a pivotal role in tumoriogenesis of human cancers, including advanced thyroid cancers. A recent study demonstrated that the increased density of TAMs was positively associated with lymph node (LN) metastasis in papillary thyroid cancer (PTC). The purpose of this study was to investigate the functional role of TMA in PTC. Method: Immunohistochemistry with anti-CD68 antibody was performed in 35 PTC patients who had LN metastasis. Co-culture of human PTC (SNU-790) cells and activated human monocytes (THP-1) by macrophage-colony stimulating factor (M-CSF, 50ng/ml) were performed and cell survival rates were analyzed by MTT assay. Result: Among 35 patients, 9 were male and 26 were females. Mean age was 48.9 ± 13.4 years and mean tumor size was 0.94 ± 0.51 cm. Twenty six (74.2%) patients had extrathyroidal invasion. Immunohistochemical staining of primary tumor showed that the expression of CD68 was presence in 3~15% of the peri- and intratumoral areas in all cases, while there was no positivity in normal thyroid tissue adjacent to tumors. In vitro stimulation of THP-1 cells with M-CSF showed significant increase of IL-10 by 7~8 fold compared to vehicle treated cells. Co-culture of SNU-790 cells with M-CSF stimulated THP-1 cells showed higher survival rates than control. Conclusions: TAMs exist in invasive human PTC specifically in intratumoral areas. Activated human monocytes, representing TAM phenotypes support tumor cell survival. Collectively, TAMs may have a functional role in tumor microenvironment of human PTC. Citation Format: Sun Wook Cho, Hye Sook Min, Seunwhan Kim, Kyung Won Kim, Do Joon Park, Young Joo Park. Tumor-associated macrophages in human papillary thyroid cancer. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr B23.