Sixty-one consecutive patients were enrolled in a randomized, controlled trial of thromboprophylaxis with a low molecular weight heparin (Seleparina, CY 216) in major abdominal oncological surgery. Thirty patients received 2 x 3,825 anti-Xa international units of CY 216 subcutaneously on the day of surgery followed by a single daily 3,825 anti-Xa international units injection for 7 days; thirty-one patients did not receive any form of prophylaxis. The occurrence of deep vein thrombosis (DVT) was detected by 125I-labelled fibrinogen leg scan. Postoperative DVT developed in 2 patients in the CY 216 group and in 11 patients in the control group (6.8% vs 35.4%, p < 0.01). Although there was a higher postoperative transfusional requirement in the group receiving CY216 (p < 0.05), the total number of patients transfused was similar in the two groups (14 vs 13). On day 1 after surgery, the two patients who later developed DVT in the CY216 group had plasma anti-Xa activity significantly lower (p < 0.01) than the remaining patients. As a good relationship was found between plasma anti-Xa activity and body weight, adoption of a personalized dosage schedule might improve efficacy of CY 216 prophylaxis.