In 40 patients believed to have deep venous thrombosis, bioscintigrams performed with I-123 fibrinogen were available for comparison with either contrast venograms or I-125 fibrinogen uptake tests. If either contrast venography or the fibrinogen uptake test was accepted as a standard for comparison, the accuracy of bioscintigraphic imaging was 90%. Most discrepancies between the procedures could be interpreted as due to heparin treatment, although bioscintigrams often were positive in patients treated with heparin for a short interval of time. Additionally, bioscintigrams provided information to distinguish venous thrombosis from other sources for local accumulation of radioactivity. Bioscintigraphy seems to be a promising noninvasive method for the detection of deep venous thrombosis of the lower extremities.