Aggressive bone resorption is well recognized in psoriatic arthritis (PsA). By using in vitro osteoclastogenesis models, consisting of unstimulated mononuclear cells from peripheral blood (PBMCs) and synovial fluid (SFMCs) of PsA patients, we demonstrated the spontaneous OC formation in both culture systems. In PBMCs the osteoclastogenesis was T cell-dependent while it was T cell- and fibroblast-dependent in SFMCs. T cells isolated from PBMCs and SFMCs of PsA patients overexpressed RANKL and TNF-alpha, and fibroblasts from SFMCs overexpressed RANKL. Thus, T cells and fibroblasts from synovial fluid through the production of osteoclastogenic cytokines could support the OC formation in PsA patients.