Coexisting ankylosing spondylitis and hereditary multiple exostoses have rarely been reported (three patients) previously. A 27-year-old man with hereditary multiple exostoses is presented as a fourth report. At the age of 15 years, the patient had multiple exostoses around the knee, ankle and shoulder joints. He was diagnosed with ankylosing spondylitis 3 years ago. The patient's sister and his 3 brothers also have multiple exostoses without any family history of spondyloarthropathy or inflammatory arthritis. The aim of this report is to discuss an interesting coexistence of these two diseases. The increasing number of reported patients who have a coexistence of these two diseases might suggest that the association of these two diseases is stronger than a coincidence.