The efficacy and safety of low-molecular weight (LMW) heparins in the prevention of thromboembolism (TE) have been shown in surgical patients. Only few studies have been performed in intensive care unit (ICU) patients who usually have many risk factors for TE. The aim of the present study was to find out, if there were any differences as regards the antithrombotic efficacy, tolerability and bleeding between two commonly used LMW heparins, dalteparin and enoxaparin. Sixty six adult mixed ICU patients to whom prophylaxis of TE was indicated on clinical basis were randomized to receive either dalteparin 5000 IU sc. or enoxaparin 40 mg sc. once a day. All patients were examined daily for TE for 7 days clinically, by chest X-ray and by above-knee ultra sound assessment using vein compression. Activated clotting time (ACT), anti-Xa-activity, and local tolerance of the sc. medication were examined daily. Serum alanine-aminotransferase (ALAT) and aspartateamino-transferase (ASAT) were determined before anti-TE-medication and on the fourth and seventh day during medication. Thirty two patients received dalteparin and 34 patients enoxaparin. The study groups were comparable according to demographic data, indication for ICU admission and risk factors for TE. Also ACT- and anti-Xa-levels, thromboplastin time (TT-SPA), activated prothrombin time (APTT) and platelet count were comparable before TE-prophylaxis. Two positive ultra sonography findings were made, one in each group. Positive findings were confirmed with phlebography. No signs of pulmonary embolism were found. The incidence of TE was 3% in both groups. On the first day dalteparin and enoxaparin increased the ACT levels to 160.7 sec ± 24.5 sd and 159.5 sec ± sd 32.3, respectively and the anti-Xa-activity to 0.32 IU ml−1 ± sd, 0.12 and 0.30 IU ml−1 ± sd 0.17, respectively. The levels were maintained throughout the study period. Few minor haematomas at the sc. injection site were found, two of them occurred in dalteparin patients and four in enoxaparin patients (NS). ALAT and ASAT changes were similar in both the groups. Both LMW heparins, 40 mg of sc. enoxaparin and 5000 IU of sc. dalteparin, once a day, caused similar increases in ACT- and anti-Xa-levels, and were as efficient in the prevention of TE in mixed ICU patients. The local tolerance, occurrence of haematomas at injection site and effect on liver enzymes were similar with both drugs.