In a randomized, prospective, double-blind multicenter trial, the effect of low-dose conventional heparin (5,000 IU/12 h) was compared to a low molecular weight (LMW) heparin fragment (5,000 antifactor Xa U/24 h). 432 patients 40 years or older undergoing elective abdominal surgery were included, 382 correctly treated. 45% had malignant diseases. The groups did not differ in risk factors. Analysis was made both on the basis of intention to treat and correct prophylaxis. No difference in results between these 2 groups was seen. Deep vein thrombosis (125I-fibrinogen) occurred in 4.3% of the low-dose heparin group and in 6.4 of the LMW heparin group. There was a significant delay in the onset of deep vein thrombosis in the LMW heparin group. Mortality, peroperative blood loss, transfusions or infectious complications did not differ. Hemorrhagic complications occurred significantly more often in the LMW heparin group (11.6%) than in the low-dose heparin group (4.6%). Significantly fewer patients experienced local injection pain in the LMW heparin group. APTT and AT III were similar in both groups, but anti-Xa activity was significantly higher in the LMW heparin group. Single daily LMW heparin injection reduced the frequency of deep vein thrombosis to the same level as low-dose heparin twice daily. The dose or administration interval of LMW heparin in this study caused significantly more bleeding complications.