Quantitative sacroiliac and lumbar spine radio-isotope (Tc-99m MDP) scans were performed in 42 patients with ankylosing spondylitis, and repeated 12 months later in 25. Clinical and laboratory assessments as well as computerised tomographic (CT) scans of the sacroiliac joints (SIJ) and lateral lumbar spine x-rays, were performed. Bone (using the L3/4 area of the lumbar spine, sacrum, SIJ's and knee) to soft tissue (ST) ratios all correlated strongly with each other. Patients with high SIJ:ST ratios had significantly greater low-back stiffness (p less than 0.05). Change in serum IgA levels correlated negatively with change in bone: ST ratios. There was no relationship between bone: ST ratios and any other clinical or laboratory variables. The change in SIJ:ST ratios correlated positively with change in CT erosion score (p less than 0.05) and negatively with change in CT ankylosis score (p less than 0.05).