Schwannosis (aberrant proliferation of Schwann cells and nerve fibers) has been reported following spinal cord injury (SCI). In this study, we examined the incidence of schwannosis following human SCI, and investigated its relationship to gliosis. We found evidence of schwannosis in 32 out of 65 cases (48%) of human SCI that survived 24 h to 24 years after injury; this incidence rose to 82% in those patients who survived for more than 4 months. Schwannosis was not observed in cases that survived less than 4 months after injury. In affected cases, it was generally noted in areas that had low immunoreactivity for glial fibrillary acidic protein (GFAP), suggesting that reduced gliosis might have contributed to the aberrant proliferation of Schwann cells following SCI. Since chondroitin sulfate proteoglycan (CSPG) has been proposed to play a role in Schwann cell/glial interaction, we performed immunohistochemical staining for CSPG to investigate its potential relationship with schwannosis. CSPG in the injured cord was generally associated with the blood vessel walls, but was also sometimes noted in reactive astrocytes. In SCI with schwannosis, CSPG staining was more prominent and confined largely to the extracellular matrix and basal lamina of proliferating Schwann cells. Our study suggests that Schwann cells, which may have been displaced from spinal roots and introduced into the injured cord through a break in the pial surface, are capable of proliferating and producing CSPG, particularly in the setting of reduced gliosis. Since CSPG has been associated with inhibition of neurite outgrowth, its increased production by aberrant Schwann cells may impair spinal cord regeneration after injury.