In two studies 267 consecutive patients with suspected leg vein thrombosis were examined by an 125I-fibrinogen uptake test (125I-FUT) and by phlebography. The ionic meglumine calcium metrizoate (Isopaque Cerebral) was used as the phlebographic contrast medium in 161 patients, and the non-ionic metrizamide (Amipaque) was used in 106. In these two groups 47 and 41 patients, respectively, had normal phlebograms as well as an initially normal 125I-FUT. After phlebography 29 (62%) of the patients who had received meglumine metrizoate had a significant rise in fibrinogen uptake, while such a rise was not found in patients examined with metrizamide. Repeat phlebography showed fresh deep-vein thrombosis in seven of the nine patients with increased uptake, indicating a complication rate of 48%. Consequently, we now use metrizamide in leg phlebogrphy; because of its expense a radioisotope test is employed as a screening procedure.