Objective: Ossification is an often encountered finding in papillary thyroid carcinoma (PTC). We hypothesized that osteogenic signaling may be related to osteogenesis of PTC. Bone morphogenic protein (BMP)-9 is the most osteogenic subtypes among BMPs. And as a cellular receptor, activin receptor-like receptor (ALK)1 has been emphasized in BMP-9 induced osteogenic signaling. We investigated the expression of ALK1 and BMP-9 and their correlation with ossification in PTC.