A 70-year-old woman experienced a balance disorder that had been worsening insidiously for 7 years, combined with mild bladder difficulties. She was a known carrier of an ATP-binding cassette subfamily D member 1 (ABCD1) mutation (c.1978C>T; p.Arg660Trp), since her son was diagnosed at the age of 5 years with X-linked adrenoleukodystrophy (X-ALD) that caused adrenal insufficiency. On examination, she had an unstable gait and an inability to tandem walk, a positive Romberg test result, and a decreased sense of vibration in her lower limbs, which suggested sensory ataxia. She had no weakness, no lower-limb spasticity, brisk deep-tendon reflexes, and normal plantar reflexes. Her brain magnetic resonance imaging (MRI) was normal, but her spinal cord MRI showed cervical posterior T2 hyperintensity at the C2-C3 level (Figure 1), suggesting chronic myelitis. An exhaustive diagnostic workup had negative results; blood markers of infectious and autoimmune diseases had normal results, including serological testing for hepatitis B virus, hepatitis C virus, HIV, syphilis, Lyme disease, and human T-lymphotropic virus 1/2 and testing for antinuclear, anti–aquaporin 4, and anti–myelin oligodendrocyte glycoprotein antibodies. All other tests also had normal results, including vitamin B9, vitamin B12, copper, and homocysteine levels; cerebrospinal fluid analysis (leukocytes, <5 cells/µL [to convert to cells per ×109/L, multiply by 0.001]; protein, 0.042 g/dL [to convert to grams per liter, multiply by 10.0]; glucose, 61 mg/dL [to convert to millimoles per liter, multiply by 0.0555]; IgG index, 0.47; no oligoclonal bands; negative bacterial culture results; and a negative herpes virus polymerase chain reaction test result); salivary gland histology; and a thoracoabdominal computed tomography scan and whole-body positron emission tomography scan.