Although heparin is administered to prevent early recurrences of venous thromboembolism (VTE) by preventing new thrombi, allowing old thrombi to attach to venous walls, and covering warfarin until it is therapeutic, heparin largely prevents late recurrences of VTE (after 5 days). The dreaded early occurrence of pulmonary embolism (PE) (on or before day 5) while waiting for the vitamin K antagonist to become antithrombotic did not occur among patients with deep venous thrombosis (DVT), who received acencoumarol alone. Fewer total recurrences resulted if a therapeutic level of heparin was reached within the first 24 hours of treatment of DVT. Fewer total recurrences also resulted if heparin was consistently maintained at therapeutic levels. A recurrent VTE within 5 days, however, occurred infrequently in those in whom heparin was not given, or therapeutic levels were delayed or not maintained. The incidence of early recurrent VTE was not lower in those who received adequate heparin than in those who did not, although the data are sparse. The true incidence of early recurrent VTE is uncertain due to the broad confidence intervals for the observed frequency of early recurrent events. Later recurrences are the norm and may be reduced by early treatment with adequate heparin.