Hepatocyte growth factor (HGF) is known to be a potent mitogen not only for hepatocytes but also for a variety of cell types including tumor cells and endothelial cells. However, still little is known about the circulating level of HGF in cancer patients. We measured the HGF level in sera in 61 breast cancer patients including 31 primary breast cancer patients and 30 recurrent breast cancer patients by an enzyme linked immunosorbent assay system. In 10 (32.3%) out of 31 primary breast cancer patients and in 25 (83.3%) out of 30 recurrent breast cancer patients, the circulating level of HGF was increased to over 0.4 ng/ml. In particular, the increase in the level of HGF was marked in patients with liver metastases (mean+/-SD: 1.286+/-0.557 ng/ml) compared to those with recurrence of other sites (mean+/-SD: 0.635+/-0.277 ng/ml). It was suggested that the increase in HGF level in sera might be associated with tumor progression, particularly with liver metastases, in human breast cancer.