An attempt has been made to improve diagnostic precision in a group of diseases associated with inflammation of the sacroiliac joints, by using a 99mTechnetium stannous pyrophosphate bone scan. Inflammation of these joints is associated with osteoblastic activity and is reflected by an increase in the uptake of radionuclide, which can be precisely quantitated. The uptake was markedly above the range of normal in patients with active ankylosing spondylitis (AS), and also in a number of patients with possible AS, psoriasis, and Reiter's syndrome. However, patients with Grade 4 radiological changes of the sacroiliac joints frequently had normal scans. This technique may be useful in the early diagnosis of sacroiliitis, and the nosological implications of the changes in patients with Reiter's syndrome and psoriasis are of interest.