Abstract:Molecular species of phosphatidylcholine containing unsaturated (i.e., monoenoic and polyenoic) 32‐ to 40‐carbon (very long chain) fatty acids (VLCFA‐PC) are present in normal human brain, the fatty acid composition changing significantly with development. There is a marked increase in the concentration and a change in the polyenoic VLCFA composition of these molecular species in brains of patients with inherited defects in peroxisomal biogenesis [Zellweger's syndrome, neonatal adrenoleukodystrophy (ALD), and infantile Refsum's disease]. In contrast, there is a marked increase in monoenoic VLCFA‐PC in X‐linked ALD whereas molecular species containing polyenoic VLCFA are minor c