The plasma of patients with inherited defects in peroxisomal biogenesis (ie, Zellweger's syndrome, infantile Refsum's disease, and neonatal adrenoleukodystrophy) shows evidence of a disturbance in the metabolism of saturated and monoenoic fatty acids with carbon chain lengths greater than 22 (VLCFA). Zellweger's syndrome plasma alone contains, in addition, increased amounts of a number of n-6 polyenoic VLCFA including 24:5, 26:5, 28:5, 30:5, and 30:6 fatty acids. These fatty acids facilitate the biochemical discrimination of Zellweger's syndrome from other related phenotypes.