An epidemiological study of hereditary neuropathy with liability to pressure palsies (HNPP) was carried out in south western Finland, with a population of 435,000. The diagnosis was established in 69 patients from 23 unrelated families through family and medical history, clinical neurological and neurophysiological examinations and with documentation of the deletion at gene locus 17p11.2 in at least one member of each family. This gave a prevalence of at least 16/100,000, which is remarkably high. However, due to the insidious nature of HNPP, most probably it is still an underestimation. This is the first population-based prevalence figure reported for HNPP. The prevalence is somewhat lower than that obtained for CMT in the same population, which agrees with the proposal that HNPP and CMT 1A are reciprocal products of the same unequal crossing-over. The clinical pictures of our patients were, in general, similar to those previously described in HNPP.