Circulating levels of hepatocyte growth factor (HGF) are increased in the early stage of an acute myocardial infarction because of arterial thrombosis. The purpose of this study was to use a new sensitive enzyme-linked immunosorbent assay to investigate whether circulating HGF is increased in patients with cerebral infarction. Circulating HGF was measured in 32 patients with cerebral infarction on admission to hospital and on days 2, 3, 7 and 14 after the onset of symptoms. Serum HGF levels exceeded the mean value +2SD (329pg/ml) measured in controls in 10 of 20 patients (50%) within 6 h after onset and in 15 of 32 patients (47%) within 24 h. Plasma D-dimer was increased in more than half of the patients with elevated HGF values. HGF levels in 16 patients who were measured serially were persistently increased throughout the study period. The results suggests that circulating HGF is a reliable early marker of cerebral infarction, and that this new sensitive HGF assay may be useful for diagnosing cerebral thrombosis.