We aim to compare the bone mineral density (BMD) in a group of patients with cervical or thoracic diffuse idiopathic skeletal hyperostosis (DISH) with that in a matched control group. We also investigated the prevalence of osteoporosis in the two groups and determined the correlation between BMD and the extent of spinal DISH. From 1999 to July 2015, 65 patients with DISH underwent dual-energy X-ray absorptiometry at our institute. The control group was matched with regard to age, sex, and body mass index to the patient group on a 1:1 basis. BMD was measured at the lumbar spine (L1-L4), femur neck, and femur total areas using dual-energy X-ray absorptiometry. The BMDs of the DISH and control groups were significantly different at the lumbar spine (L1-L4) and the femur neck (p = 0.005, 0.001). The rates of patients with osteopenia and osteoporosis were lower in the DISH than in the control group for the lumbar spine (L1-L4) (p = 0.05). A positive correlation was observed between the lumbar spine (L1-L4) BMD and the number of spine levels affected by DISH (p = 0.04). The BMDs of the lumbar spine and femur neck were found to be higher in the DISH group than in a matched control group, when patients with lumbar DISH involvement were excluded. The rates of osteopenia and osteoporosis tended to be lower in the DISH group than in the control group. Lumbar spine BMD is significantly correlated with the number of spine levels affected by DISH.