To test the efficacy and safety of low molecular weight heparin (LMWH) fractions in thromboembolic diseases, 2 pilot studies were performed. The aim of the first was to test the antithrombotic efficacy of the LMWH PK 10169 (Pharmuka Laboratories) in 9 patients (group I) with acute pulmonary embolism documented by pulmonary angiography and scintiscan. The second study was intended to test the safety of the same LMWH fraction in a group (group II) of 30 patients at high risk of bleeding but also requiring antithrombotic therapy. In the 2 groups, the antithrombotic efficacy of PK 10169 was found to be excellent: no recurrence of pulmonary embolism was noted in group I and no thrombotic event occurred in group II. Neither side effects nor major bleeding complications occurred in either group. However, 1 patient in group II had a moderate bleeding in relation to a transient overdosage, and 3 others exhibited minor bleeding which did not necessitate dosage adjustment. In these studies, PK 10169, at the dosage used, proved an effective antithrombotic therapy with few side effects even in a group of patients considered to be at high risk of bleeding.