2136 patients in general surgery, gynaecology and urology were investigated by the 125I-fibrinogen-uptake-test for detection of postoperative deep vein thrombosis (DVT). They received at random one of low-dose heparin, dihydroergotamine, the fixed combination of both drugs (Heparin-Dihydergot), low molecular weight dextran and acetylsalicylic-acid (ASS). When DVT was detected repeated lung perfusion scintigraphies were carried out for diagnosis of embolic pulmonary perfusion defects. Results demonstrate the outstanding effect of Heparin-Dihydergot, which is not only 2-3 times better than the anti-thrombotic standard low-dose heparin but also eliminates almost completely the risk of postoperative embolism. The preventive efficacy of ASS and dextran must be considered to be poor and not comparable to that obtained when using heparin, dihydroergotamine or the combination. Now Heparin-Dihydergot is the new standard with which all prophylactic procedures should be compared.