Amyotrophic lateral sclerosis (ALS) is a progressive, lethal neurodegenerative disease without any effective therapy. To evaluate the potential of wild-type bone marrow (BM)-derived stem cells to modify the ALS phenotype, we generated BM chimeric Cu/Zn superoxide dismutase (SOD1) mice by transplantation of BM cells derived from mice expressing green fluorescent protein (GFP) in all tissues and from Thy1-YFP mice that express a spectral variant of GFP (yellow fluorescent protein) in neurons only. In the recipient cerebral cortex, we observed rare GFP+ and YFP+ neurons, which were probably generated by cell fusion, as demonstrated by fluorescence in situ hybridization (FISH) analysis, suggesting that this phenomenon is not limited to Purkinje cells. GFP-positive microglial cells were extensively present in both the brain and spinal cord of the affected animals. Completely differentiated and immature GFP+ myofibres were also present in the heart and skeletal muscles of SOD1 mice, confirming that BM cells can participate in striated muscle tissue regeneration. Moreover, wild-type BM chimeric SOD1 mice showed a significantly delayed disease onset and an increased life span, probably due to a positive 'non-neuronal environmental' effect rather than to neuronogenesis. This improvement in SOD1-G93A mouse survival is comparable with that previously obtained using some safer pharmacological agents. BM transplantation-related complications in humans preclude its clinical application for ALS treatment. However, our data suggest that further studies aimed at improving the degree of tissue chimerism by BM-derived cells may provide valuable insights into strategies to slow ALS progression.