In chronic sinusitis, although the pathogenesis in the sinus mucosa has been widely investigated, the pathogenesis in the underlying bone remains poorly understood. As a first step in investigating the pathogenesis in sinus bone, sinus-bone models should be constructed by co-culturing of osteoblasts and osteoclasts in a sinus. However, human osteoclast cell lines derived from a sinus have not been established. Based on previous methods of another osteoclast culture, we hypothesized that human osteoclasts in sinuses could be established in vitro by co-culture with human osteoblasts in the sinus. The aim of this study was to differentiate human bone marrow cells into osteoclasts in a sinus, which is identified by the formation of absorption lacuna and positive cytochemical staining for tartrate-resistant acid phosphatase (TRAP). Differentiation was induced in the co-culture system by treatment with medium containing 1.25.(OH)2D3, G-CSF and dexamethasone. Thirty days after culturing, human bone marrow cells were differentiated to a hypertrophic state, as evidenced by their sizes and the demonstration of osteoclast-specific abilities. Our results indicate that sinus-osteoclasts can be cultured from human bone marrow cells, and that osteoclasts in a sinus may be a useful model for investigating sinus-bone remodeling, especially in terms of bone resorption, in the sinus system.