To clarify the trophic mechanism of residual anterior horn cells affected by sporadic amyotrophic lateral sclerosis (SALS) and familial ALS (FALS) with superoxide dismutase 1 (SOD1) mutations, we investigated the immunohistochemical expression of hepatocyte growth factor (HGF), a novel neurotrophic factor, and its receptor, c-Met. In normal subjects, immunoreactivity to both anti-HGF and anti-c-Met antibodies was observed in almost all anterior horn cells, whereas no significant immunoreactivity was observed in astrocytes and oligodendrocytes. Histologically, the number of spinal anterior horn cells in ALS patients decreased along with disease progression. Immunohistochemically, the number of neurons negative for HGF and c-Met increased with ALS disease progression. However, throughout the course of the disease, certain residual anterior horn cells co-expressed both HGF and c-Met with the same, or even stronger intensity in comparison with those of normal subjects, irrespective of the reduction in the number of immunopositive cells. Western blot analysis revealed that c-Met was induced in the spinal cord of a patient with SALS after a clinical course of 2.5 years, whereas the level decreased in a SALS patient after a clinical course of 11 years 5 months. These results suggest that the autocrine and/or paracrine trophic support of the HGF-c-Met system contributes to the attenuation of the degeneration of residual anterior horn cells in ALS, while disruption of the neuronal HGF-c-Met system at an advanced disease stage accelerates cellular degeneration and/or the process of cell death. In SOD1-mutated FALS patients, Lewy body-like hyaline inclusions (LBHIs) in some residual anterior horn cells exhibited co-aggregation of both HGF and c-Met, although the cytoplasmic staining intensity for HGF and c-Met in the LBHI-bearing neurons was either weak or negative. Such sequestration of HGF and c-Met in LBHIs may suggest partial disruption of the HGF-c-Met system, thereby contributing to the acceleration of neuronal degeneration in FALS patients.