Human Schwann cells (SCs) can be isolated and expanded with mitogens using cell culture techniques. These cells have been demonstrated to promote axonal regrowth in both the central and peripheral nervous system. Primary rat SCs can be immortalized with long-term exposure to mitogens. Transplantation of these cells into an autogenic host sciatic nerve results in the formation of tumors. Human cells are, in general, relatively more resistant to malignant transformation, but any potential risk for inducing tumor after transplantation of SCs in humans must be excluded. In this study, the malignant potential of mitogen expanded human SCs injected into the sciatic nerve of immunodeficient mice was investigated. Human SCs were isolated from human peripheral nerves and placed in cell culture, expanded with mitogens (heregulin and forskolin) for many passages (0-6 times), and then injected within the sciatic nerve of Severe Combined Immunodeficient (SCID) rat or mice. As a positive control for tumor formation in this xenograft model, human glioma cells were also injected. The proliferation index (PI) of the human SCs gradually decreased with each passage in cell culture. SC purity remained stable until the 6th passage, and then decreased significantly for older passages, so that the cultures were over-grown with fibroblasts. The incidence for rat or human glioma cells to induce tumors was 100% and 92%, respectively. In contrast, there was no tumor induced by human primary or mitogen expanded SCs. Demyelination, remyelination and formation of connective sheath at the injection site were observed in some cases after injection of the human SCs. Thus, mitogen-expanded human SCs do not produce tumors when transplanted in vivo, which suggests that these cells are safe, and deserve further study towards their use in clinical transplantation.