The X-linked adrenoleukodystrophy (ALD) gene was identified recently and is predicted to encode a 745-amino-acid peroxisomal membrane protein. Strategies have been designed for the search for mutations in the ALD gene in patients. Several mutations have now been found and it seems that many different mutations are responsible for ALD. There is no straightforward correlation between genotype and phenotype since the same mutation can cause different ALD phenotypes in the same family. However, once a mutation has been found in a family, it can be traced in all at-risk individuals of that family, both post- and prenatally, without the need for very long-chain fatty acid (VLCFA) analysis. Segregation analysis with extragenic and intragenic polymorphisms may remain useful in families where mutation analysis is not possible for practical reasons; VLCFA analysis and measurement of the peroxisomal beta-oxidation with C26:0 fatty acid as a substrate will remain the alternative. We also briefly discuss the possibilities of DNA diagnosis for other peroxisomal disorders.