Hereditary motor and sensory neuropathy type I (HMSNI), also known as Charcot-Marie-Tooth disease type 1 (CMT1), has been shown to be genetically heterogeneous. A major gene maps to chromosome 17 (CMT1A). A set of loci, D17S122, D17S125, and D17S124, show tight linkage to the CMT1A locus, and a duplication of D17S122 has been detected in some families. We show that the locus D17S122 is duplicated in affected individuals from 7 informative families with HMSNI. The duplication was demonstrated either by differences in hybridization densities between two bands of a restriction fragment length polymorphism or by the presence of all three alleles. No normal individual had the duplication. A single recombinant exists between the MspI polymorphism of D17S122 and the duplicated band, suggesting that the duplication is of considerable size. Patients with HMSN type II do not show the duplication. These findings will have considerable impact on the diagnosis of chronic demyelinating neuropathies, in patients with or without similarly affected relatives.