The results of streptokinase and heparin treatment are compared in a 4-year prospective study with special reference to preservation of high valvular venous function. An objective assessment was based upon phlebographic examinations before, during and 1-2 months after therapy. Complete lysis was demonstrated in 44% of high thromboses treated with streptokinase and in 6% treated with heparin. Retrograde phlebography revealed normal function of the proximal femoral valves in 92% of streptokinase-treated high thromboses, compared with 13% of those treated with heparin. These phlebographic results were considered to be a valid prognostic indicator of the eventual development of the postthrombotic syndrome. Allergic reactions were seen in 39% and minor haemorrhagic complications in 18% of the streptokinase-treated cases. The therapeutic benefit of streptokinase therapy in this study was found to outweigh any disadvantages incurred by observed complications.