A simple, accurate test with no morbidity and effective for the detection of urinary tract obstruction in patients with normal and impaired renal function would be of significant clinical use. We herein evaluate the ability of 99mtechnetium diethylenetriaminepentaacetic acid to detect the presence or absence of obstruction. Of 107 patients who underwent recent studies 31 had confirmation of the presence or absence of obstruction by excretory urography, retrograde pyelography or other definitive procedures. Patients were studied in the posterior projection wtih perfusion images after intravenous injection of 20 millicuries 99mtechnetium diethylenetriaminepentaacetic acid. Static images of the urinary tract were obtained from 1 to 30 minutes after injection with delayed images up to 24 hours. Obstruction were defined as abnormal retention of activity in the collecting system persisting in the delayed images. Diethylentriaminepentaacetic acid identified correctly 13 of 13 patients proved to have obstruction and 17 of 18 without obstruction. One falsely positive result was owing to pyelocaliectasis. These data indicate a sensitivity of 100 per cent, specificity of 94 per cent and accuracy of 97 per cent.