Hereditary neuropathy with liability to pressure palsies (HNPP) is commonly associated with a 1.5-megabase deletion on chromosome 17p11.2-12. We analyzed the phenotypic expression of the deletion in 39 HNPP patients from 16 families carrying the deletion. Two-thirds of the individuals had episodes of acute mononeuropathy, often involving nerve territories of the upper limbs or brachial plexus; however, 41% of affected subjects were unaware of their disease, and 25% were almost or totally free of symptoms; one-third complained of chronic symptoms and four older patients had a picture of polyneuropathy. Electrophysiologic abnormalities differed among affected subjects, ranging from conduction abnormalities localized at common entrapment sites to diffuse conduction slowing, usually more evident at entrapment sites; patients from one family had preeminent proximal involvement. The spectrum of phenotypic expression of deletion-associated HNPP appears to be broader than previously thought. The prevalence of the disease is probably underestimated, and the availability of molecular diagnosis should increase disease detection.