This review discusses the problem of deep vein thrombosis (DVT) after operation and identifies three levels of risk of DVT: low (< 10 per cent), moderate (10–40 per cent) and high (40–80 per cent). Special emphasis is placed on the most recent prophylactic treatment, low molecular weight heparins (LMWHs), particularly enoxaparin. Several LMWHs are now available, but they difer slightly and each must be evaluated on its own merits. In general, however, LMWHs are both efective and safe in those patients at moderate or high risk of DVT. Thromboprophylaxis is cost effective when analysed using healtheconomic methodology.