Astrocyte proliferation is tightly controlled during development and in the adult nervous system. In the present study, we find that a high-molecular-weight (MW) form of the glycosaminoglycan hyaluronan (HA) is found in rat spinal cord tissue and becomes degraded soon after traumatic spinal cord injury. Newly synthesized HA accumulates in injured spinal cord as gliosis proceeds, such that high-MW HA becomes overabundant in the extracellular matrix surrounding glial scars after 1 month. Injection of hyaluronidase, which degrades HA, into normal spinal cord tissue results in increased numbers of glial fibrillary acidic protein (GFAP)-positive cells that also express the nuclear proliferation marker Ki-67, suggesting that HA degradation promotes astrocyte proliferation. In agreement with this observation, adding high- but not low-MW HA to proliferating astrocytes in vitro inhibits cell growth, while treating confluent, quiescent astrocyte cultures with hyaluronidase induces astrocyte proliferation. Collectively, these data indicate that high-MW HA maintains astrocytes in a state of quiescence, and that degradation of HA following CNS injury relieves growth inhibition, resulting in increased astrocyte proliferation. © 2005 Wiley-Liss, Inc.