A prospective, randomized, double-blind trial concerning prophylaxis of thrombosis in 82 patients undergoing hip fracture surgery was conducted to compare a new low-molecular-weight heparin (Fragmin) with a placebo. Deep venous thrombosis (DVT) was detected by I125-fibrinogen uptake test followed by ascending phlebography when positive. Sixty-eight patients completed the study and a 50% reduction in the incidence of DVT was demonstrated: 9/30 (30%) patients in the treatment group and 22/38 (50DVT. This significant difference was achieved by one daily dose of 5.000 IU Fragmin subcutaneously, commencing preoperatively and continuing for six days. No differences in bleeding nor other complications in the two groups were observed. Fragmin administered once daily offers an effective and safe prophylaxis of thrombosis in hip fracture surgery.