Twenty out of 102 patients studied with 125I-fibrinogen had positive test results. Of these, acute thrombophlebitis was confirmed by radiopaque venography in 12 and by clinical evaluation and subsequent hospital course in eight. None of the 30 radiopaque venograms performed in patients with negative fibrinogen-uptake tests revealed evidence of acute thrombophlebitis. Most significantly, 85% of the positive tests were evident within 24 hr after administrationof the radiopharmaceutical, thus indicating the clinical value of this procedure. Forty-two percent of 24 patients suspected of acute thrombophlebitis and 50% of 14 patients with documented pulmonary emboli had positive fibrinogen tests. Anticoagulation therapy did not prevent a positive 125I-fibrinogen result. Followup studies, conducted 1-6 months after injection of 125I-fibrinogen, showed no evidence of hepatitis in any of the recipients.