The value of compression sonography was assessed to reduce the number of phlebographies otherwise necessary when deep venous thrombosis (DVT) is suspected among hospital patients. Compression sonography was used to study 119 prospective hospital patients who were suspected of having DVT of the lower extremity. The results were compared to those by phlebography. There were 44 DVTs detected by phlebography; ten of these were located only in the calf, below the knee. Of the remaining 34 femoropopliteal DVTs 33 were detected by the compression sonography technique. It is concluded that the use of compression sonography for primary investigation of suspected DVT reduces the number of patients who require phlebography to those whose results by compression sonography are negative. In our study, this would have represented a decrease of 28%, corresponding to a reduction of the total diagnostic costs by 10%.