Beside complex structures such as endoplasmic reticu lum or mitochondria that electron microscopy reveals in human cells, there are organelles bounded by a single membrane; some of these vesicles contain hydrogen-peroxide-generating enzymes and were therefore called 'peroxi somes' to reflect their role in peroxide metabolism. It was later found that enzymes catalyzing the p-oxidation of fatty acids were located not only in mitochondria but also in per oxisomes; in fact the most important metabolic role of the peroxisome seems to be shortening of very-long-chain fatty acids (>22 carbon atoms) and oxidizing di-and trihydroxycoprostanic acids and pristanic acid, a metabolite of phyta nic acid [1], Thus peroxisomes contain enzymatic activities for P-oxidation of fatty acids that are distinct from the mito chondrial pathway.Peroxisomes Papini et al. [8] report the dermatological findings in a patient with X-linked ALD and the metabolic disturbances detectable in the skin sur face lipids.X-linked ALD is a rare peroxisomal disorder, affecting 1/20,000 males. The disease is characterized by an accumu lation of very-long-chain fatty acids because of impaired shortening in peroxisomes. Diagnosis of X-linked ALD is based on the demonstration of high serum levels of verylong-chain fatty acids. Clinical manifestations are central nervous system demyelination and adrenal insufficiency. However, striking variation of the clinical phenotype exists. The gene locus for ALD has been mapped to the ter Prof. J. Ulrich is acknowledged for kindly reviewing the manuscript.