We studied the effect of argatroban, a new selective thrombin inhibitor, on the haemostatic system in seven patients with acute ischaemic stroke (the argatroban group). Argatroban was infused continuously at 2.5 mg/h for the first 48 h, and then 10 mg of argatroban was infused over 3 h twice a day on days 3-7. The placebo group consisted of six acute ischaemic stroke patients. As a combination therapy, intravenous administration of glycerol was also performed at the same time in five of the seven patients in the argatroban group and in four of the six patients in the placebo group. D-dimer levels were measured by a latex photometric immunoassay that allowed immediate quantitative assessment. The D-dimer levels of our 13 patients with acute ischaemic stroke were raised at the time of admission (day 1) and 69% of the values were above the 97th percentile ( > 500 ng/ml) in healthy subjects. D-dimer levels were significantly reduced in the argatroban group on days 2 and 7 after admission when compared with the placebo group (day 2: P = 0.032; day 7: P = 0.046). Thus, haemostatic activation occurred in acute ischaemic stroke was effectively blocked by argatroban.