A comparative study of 27 patients with suspected deep vein thrombosis was carried out using 99mTc-labeled red blood cells and conventional X-ray venography. In 22 of the 27 patients with results of the two methods were in agreement; in 12 cases thrombosis was present and in 10 cases it was absent. In the remaining five cases the results of the two methods were at variance. Venography was negative in one patient who had all the clinical signs and symptoms of thrombosis. Scintigraphy was negative in four patients, in three of whom venography showed only minute thrombi. The use of Tc-labeled red blood cells is considered an acceptable alternative for the detection of deep vein thrombosis.