Patients with severe ankylosing spondylitis of long duration often have spinal osteoporosis secondary to ankylosis and immobility. Bone mineral density of defined regions of the lumbar spine, femoral neck, and carpus was measured in 25 men who met accepted diagnostic criteria for ankylosing spondylitis but had early disease, with normal mobility and no, or very minor, radiological evidence of lumbar spine involvement. Compared with age-matched male controls, patients with ankylosing spondylitis had a significantly lower hydroxyapatite density in the lumbar spine (mean [SD] 0.82 g/cm2 [0.12] vs 0.91 g/cm2 [0.11]) and in the femoral neck (0.83 g/cm2 [0.11] vs 0.92 g/cm2 [0.11]). There was no significant difference in carpal bone mineralisation density. The pattern of bone loss in these patients indicates early loss of trabecular bone in ankylosing spondylitis, possibly from a systemic cause, but biochemical indices of calcium turnover were similar in patients and controls.