Histological examination was performed on the cervical spinal cord from 13 horses with chronic cervical compressive myelopathy of 4 to 29 months duration. Structural alterations were correlated with clinical features. At the level of compression, the spinal cord was grossly deformed. Histological alterations included nerve fibre swelling and degeneration, occasional spheroids, astrocytic gliosis, increased macrophage activity and increased perivascular collagen. Myelin degeneration or loss at the level of the compressive lesion was greatest in the ventral and lateral funiculi and less consistently present in the dorsal funiculi. Asymmetry of lesions in the dorsal funiculi was associated with asymmetry of clinical signs in 5 horses. Histological alterations in areas of Wallerian degeneration were similar to that at the level of spinal cord compression, except that perivascular collagen was not increased. Wallerian degeneration was present cranial to the compressed site in the superficial portions of the lateral funiculi and in the middle of the dorsal funiculi. Caudal to the compressed site it was present in the ventral funiculi adjacent to the ventral median fissure and in the middle of the lateral funiculi. Deformation of the spinal cord did not correlate with the severity or duration of clinical signs but was positively correlated with the amount of perivascular collagen increase. The amount of nerve fibre swelling was not correlated with the severity of clinical signs but was negatively correlated with their duration. A rapid loss of nerve fibres apparently occurred early in the course of compression, since there was a marked decrease in the amount of nerve fibre swelling and Marchi stained degenerating myelin with increasing clinical duration.(ABSTRACT TRUNCATED AT 250 WORDS)