Peroxisomes in higher animals had been considered as a fossil organelle. But, the discovery of peroxisomal fatty acid beta-oxidation system in 1976 revived a great deal of interest. In the last decade, peroxisomes have been shown to have an important role in the lipid metabolism. In accord with this progress, the presence of peroxisome diseases has been established. The peroxisomal disorders recognized at present comprise 12 different diseases, with neurological involvements in 10 of them. Recent studies have identified at least eight complementation groups in peroxisome-deficiency disorders, and indicated that currently used clinical categories do not represent distinct genotypes.