Plasmacytomas are tumors composed of plasma cells of variable maturity, which are histologically identical to those seen in multiple myeloma. Ankylosing spondylitis is a chronic inflammatory disease, probably resulting from the interaction of a genetic predisposition involving HLA-B27 with an environmental event such as enteric bacterial infection. Multiple myeloma has been intermittently reported in patients with ankylosing spondylitis. It has been proposed that the protracted stimulation of immunocytes by inflammatory lesions on the mucosal surfaces of the gastrointestinal, respiratory tracts may be implicated in the pathogenesis of multiple myeloma in some patients. We observed a 23 year old male patient with a history of plasmacytoma who subsequently developed ankylosing spondylitis. He was diagnosed as plasmacytoma 4 years ago and took a radiation therapy. There was no previous report of ankylosing spondylitis following plasmacytoma. The relationship between two diseases is uncertain until now and further study should be needed.