Venography was performed on 44 patients by injecting radionuclide into a foot vein below an ankle tourniquet and performing rapid-sequence scintiphotography of venous flow in the calves, thighs, and abdomen. A lung scan was also obtained with use of human serum albumin microspheres labeled with Tc 99m. Standard contrast venography was done on 34 of the limbs. There was good correlation between the results from contrast and those from radionuclide venography (89%). Thirteen (39%) of the 33 patients with an abnormal radionuclide venogram had an abnormal lung scan, indicating pulmonary emboli. Of six patients who had pulmonary embolism but clinically silent legs, five had abnormal vein scans. Seven patients with abnormal venograms had abnormal lung scans, suggesting pulmonary emboli, even though they were asymptomatic in both areas. This method seems ideal for detection of the "silent embolus" and is of particular value in understanding the natural progress of the thrombotic process.