Arthritis of the sacro-iliac joints is a fairly common condition occurring either in isolation or as a part or pelvospondylitis. Because of the difficulty of establishing a diagnosis before X-ray changes have appeared, various radionuclide scanning techniques have been developed to make possible an early diagnosis. Opinions have differed as to the reliability of radionuclide methods using "bone seeking" radiopharmaceuticals. Patients with clinically and/or radiologically proven sacro-iliitis were imaged by means of 99m-Tc-EHDP. An S.I. index was calculated as the ratio of the maximum activity of the sacro-iliac joints to the minimum activity of the iliac wings. The accuracy of the method was very good, and it is recommended as a supplement to clinical examination and X-ray studies.