X-Adrenoleukodystrophy (X-ALD) is an inherited peroxisomal disorder of deficient catabolism of very long-chain (VLC) fatty acids with resulting neuroinflammatory demyelinating disease. Our recent documentation of nitric oxide (NO)-mediated increase in VLC fatty acid levels in glial cells and demonstration of greater increase of VLC fatty acids levels in the inflammatory region (plaque) of X-ALD brain as compared to the normal-looking region away from the plaque prompted us to investigate the possible involvement of NO in the pathophysiology of X-ALD. Herein we provide evidence of the expression of inducible nitric oxide synthase (iNOS) in the CNS of X-ALD patients. In situ hybridization demonstrated that iNOS mRNA was present in brain tissues from X-ALD patients but not in normal controls. Double-labeling immunofluorescence studies using cell-specific markers confirmed that iNOS-expressing cells in the CNS of X-ALD were astrocytes and microglia/macrophages. Finally, antibodies against nitrotyrosine strongly immunoreacted with tissues from the center of the plaque region of X-ALD brains suggesting the presence of the NO reaction product nitrotyrosine in the CNS of X-ALD. Taken together, these results demonstrate that iNOS is expressed in the brains of patients with X-ALD and may contribute to the pathogenesis of the disease.