We evaluated the antithrombotic efficacy of the low molecular weight heparin (LMWH) fraction PK 10169 in nine consecutive patients with acute pulmonary embolism documented by pulmonary angioscan and angiography. Therapy with PK 10169 was initiated by an i.v. bolus of 0.5 mg/kg, followed by a continuous intravenous infusion during the first 10 days; the drug was then given subcutaneously twice daily during the following 15 days. The dosage of PK 10169 was adjusted by daily measurements of anti-Xa and anti-IIa activities using amidolytic methods. For a dosage ranging from 1.4 to 4.1 mg/kg per day during the i.v. period and from 0.7 to 3.5 mg/kg per day during the s.c. period, the anti-Xa activity ranged from 4 to 8.7 PK U/ml and from 4.5 to 7.2 PK U/ml respectively. Clinical improvement was observed in all the patients and was consistent with progressive reperfusion evaluated by successive angioscans. No recurrence of pulmonary embolism occurred. No deleterious hemorrhagic side-effects were observed, even in two patients at high risk of bleeding. In this pilot study, the LMWH fraction PK 10169 proved to be an effective anticoagulant therapy during the first three weeks after pulmonary embolism in man.