In their recent article on the long-term clinical course of acute deep venous thrombosis, Prandoni and colleagues [1] found a strong association between ipsilateral recurrent deep venous thrombosis and the post-thrombotic syndrome. The authors concluded that "because recurrent venous thrombosis strongly predicted the development of the post-thrombotic syndrome, the prevention of recurrent deep venous thrombosis might be a key to decreasing the incidence of this condition." We do not believe that the authors' data allow this conclusion. The study shows only an association between the two conditions and does not provide information on the temporal relation.

Common sense suggests that the greater the number of thrombotic episodes in the same vein, the greater are the chances of developing the post-thrombotic syndrome. Common sense, however, may also suggest a completely different interpretation of the association observed by Prandoni and colleagues. The post-thrombotic syndrome, albeit mild and not yet clinically evident, might develop after the first thrombotic episode, causing venous stasis and thereby increasing the risk for ipsilateral thrombotic recurrences. Prandoni and colleagues' observation that the incidence of the post-thrombotic syndrome was higher than that of recurrent deep venous thrombosis in the first years of follow-up seems to favor the hypothesis that the post-thrombotic syndrome preceded rather than followed the episodes of recurrent thrombosis. On the basis of the available data, however, it is impossible to know the real temporal relation between the two conditions. The problem is of great importance because it will influence the clinical implications of the observed association. Instead of trying to prevent thrombotic recurrences in order to decrease the incidence of the post-thrombotic syndrome, as suggested by Prandoni and colleagues, we might have to prevent the post-thrombotic syndrome in order to decrease the incidence of recurrent deep venous thrombosis.