A nine-year-old gelding pony presented following a 3.5 month history of progressive ataxia and muscle wasting. The referring veterinarian diagnosed equine protozoal myeloencephalitis (EPM) on the basis of CSF and serological studies. Treatment was begun, without clinical improvement. The horse exhibited severe, generalized muscle wasting, and C1-6 neurological signs, though cervical radiographs, cervical myelogram, and brain and cervical CT scans revealed no abnormalities. CSF tap revealed an increased nucleated cell count that consisted overwhelmingly of small lymphocytes, some with bizarre nuclei. Necropsy revealed a raised, lacy white infiltration within the dorsal spinal segments of C1 and C2 and thickened ventral nerve roots, consistent with a diagnosis of lymphoma. Histology and immunophenotyping, however, revealed instead evidence of a lymphocytic, severe, diffuse, chronic polyradiculoneuritis and meningitis with a degenertaive myelopathy, axonal swelling, and Wallerian degeneration. Serum samples submitted for a Lyme western blot and KELA revealed a high level of antibody to Borrelia burgdorferi. Spinal samples are currently being examined with PCR for definitive confirmation of this diagnosis, which would be one of the first pathologically-documented cases of equine spinal neuroborreliosis.