Familial expansile osteolysis is a rare bone dysplasla which is transmitted as an autosomal dominant trait in a large kindred in Northern Ireland. The gene which causes the disease shows tight linkage with several polymorphic markers on chromosome 18q with a maximum lod score of 11.53 at a recombination fraction of 0.00 with D18S64. The gene is flanked by D18S35 and D18S61 and is located at chromosome 18q21.1–q22. Mapping a new locus for a gene involved in regulation of bone metabolism may also have implications in the study of Paget's disease of bone which is a common related bone dysplasla.