A 54-year-old man developed progressive spastic paraparesis beginning 2 weeks after a back injury caused by a subacute compressive thoracic myelopathy attributable to a post-traumatic arachnoid cyst. Three to 18 months after surgical decompression of the thoracic arachnoid cyst, the patient developed a diffuse predominantly upper motor neuron syndrome characterized by spastic quadriparesis, pseudobulbar paresis, and pseudobulbar affect. Retrograde corticospinal tract degeneration and upper motor neuron death after spinal cord injury is recognized. This case suggests that focal upper motor neuron injury can occasionally precipitate diffuse upper motor neuron dysfunction.