The authors evaluated a group of 78 patients where in addition to radionuclide venography (RNV) also X-ray phlebography was used (RTGF) within a time interval of not more than 72 hours. The results of 102 examinations were compared, 276 areas were correlated at six different sites. Agreement was recorded in 249 instances (90%), a false negative finding in 23 (8%) and a false positive finding in 4 (2%). Sensitivity was low in the area of the calf (55%), high in the femoral area, iliac area and in the area of the inferior vena cava (92-100%). The specificity was in all areas, incl. the calf, relatively high (94-100%). The investigation revealed the disadvantages of RNV--a low reliability in the area of the calf, the impossibility to detect small and parietal thrombi, the inability to assess the "activity and age" of the thrombus, the more difficult interpretation in insufficiency of venous perforators, and advantages of RNV--a great reliability at higher parts of the venous system and after administration of radiopharmaceutical preparations (RF) into peripheral veins, the possibility of simultaneous examination of pulmonary perfusion without further administration of RF, a low failure rate of intravenous administration of RF (less than 2%); RF do not irritate the venous endothelium and therefore thrombosis cannot develop; in case of paravenous escape of RF local inflammations do not develop, the radiation load is low, there is a small probability of allergic reactions, the method is non-invasive, it can be frequently repeated and used for the follow-up of treatment.