Two hundred and seventy patients over 65 years were included in a placebo-controlled randomized double-blind trial to determine whether a small dose of a low molecular weight (LMW) heparin prevents the occurrence of deep vein leg thrombosis (DVT) diagnosed by 125I fibrinogen scanning. LMW heparin (60 mg daily) significantly reduced the frequency of DVT from 9 to 3 percent (p = 0.03). Adverse drug reactions did not differ significantly between the 2 groups, except for the injection site hematomas that were more frequent in the LMW heparin group. In conclusion, LMW heparin appears of value in preventing the occurrence of DVT in an unselected elderly in-patient population.