There is minimal information on bone Histomorphometric characteristics in ankylosing spondylitis. We here report a case of a 36-year-old Japanese male that suffered from lumbago and could not gaze in the forward direction. Ultimately, a diagnosis of ankylosing spondylitis was made, and vertebroplasty was performed to correct the third lumbar spine. Histomorphometry of the iliac bone showed reduced bone volume parameters (bone volume, and trabecular thickness and width) than reference values. In addition, bone formation parameters (osteoid thickness and osteoblast surface per bone surface) and bone resorption parameters (eroded surface per bone surface and osteoclast number per bone surface) were also lower than reference values, indicating low bone turnover. By contrast, there was not a clear trend in bone resorption markers: bone- pecific alkaline phosphatase (17 U/l) was normal, TRACP-5b (136 mU/dl) was slightly lower, urinary N-terminal telopeptide (45.3 nmol BCE/mmol Cr) was normal, and deoxypyridinoline (9.1 nM/mM Cre) was higher than reference values. However, there was deficiency in 25-hydroxy vitamin D (25-OH-D; 14.4 ng/ml). This case highlights the rare possibility of performing bone histomorphometry, and indicates that a low bone volume and low bone turnover (in both bone formation and resorption) are characteristics of ankylosing spondylitis, although bone formation markers (bone-specific alkaline phosphatase) and bone mineral density are within the normal range. The possibility of a serum 25-OH-D deficient status in ankylosing spondylitis should be further considered.