X-linked agammaglobulinaemia (XLA) is an inherited disorder characterised by a lack of circulating B-cells and antibodies. While the gene involved in XLA has not yet been identified, the locus for the disorder is tightly linked to the polymorphic marker DXS178, which maps to Xq22. Fabry disease is an X-linked recessive disorder caused by a deficiency in the lysosomal enzyme alpha-galactosidase A. The gene encoding this enzyme has been characterized and also maps to Xq22. Using pulsed field gel electrophoresis we have constructed a long-range restriction map that shows that the alpha-galactosidase A gene (GLA) and DXS178 lie no more than 140 kb apart on a stretch of DNA containing a number of putative CpG islands. We have also isolated yeast artificial chromosome (YAC) clones that confirm this physical linkage. The localisation of DXS178 near the alpha-galactosidase A gene will facilitate carrier detection in Fabry families using restriction fragment length polymorphism (RFLP) analysis. The identification of a number of CpG islands near DXS178 also provides candidate locations for the gene responsible for XLA.