To evaluate the clinical and socio-economic importance of the postthrombotic syndrome (PTS), the following epidemiologic parameters are to be known: The incidence of deep vein thrombosis (DVT) in the population, the incidence of PTS after DVT, the prevalence of the PTS, the socio-medical consequences and the mortality. The DVT-incidence in the literature is about 3/1000 per year in the adult population. In our own follow-up study we found a global PTS-incidence of 40% (10% with, 30% without ulcer) 13 years after DVT. The PTS-incidence was correlated with the initial DTV-extent. The risk of PTS after 3- and 4-level DVT is significantly reduced by a successful fibrinolysis in the acute stage (p = 0.01). The mortality of patients with PTS is 3 times higher than in the population. 4% of the patients with PTS get disabled. To our knowledge an adequate study with direct assessment of the PTS-prevalence does not exist, but the PTS-prevalence can be deduced from large epidemiologic studies, such as the Basel-Study, in which the global chronic venous insufficiency is assessed. It amounts in the adult population between 0.5 to 1% for the PTS with and 3 to 5% for the PTS without ulcer.