Peripheral myelin protein-22 (PMP22) is expressed in myelinating Schwann cells and shows significant homology to murine growth arrest-specific gene gas3. Charcot-Marie-Tooth disease type 1a (CMT1a) is a common hereditary demyelinating neuropathy. Recently it was demonstrated that the gene for PMP22 is duplicated in CMT1a patients. A gene dosage mechanism has been postulated to cause CMT1a. According to this hypothesis, the increase in copy number of PMP22 gene would lead to an elevated expression of PMP22 and thereby cause the demyelinating phenotype of CMT1a. In the present communication we analyzed PMP22 mRNA and protein expression in sural nerve biopsies from CMT1a patients and normal controls. We show that PMP22 mRNA expression in CMT1a is not uniform. We found both elevated as well as normal PMP22 mRNA levels in patients. Interestingly, the highest PMP22 mRNA level was found in the least affected patient. In contrast to the mRNA levels, PMP22 was clearly reduced in all CMT1a patients as shown by immunohistochemistry. Thus the CMT1a phenotype may not be strictly correlated with increased PMP22 mRNA and protein expression. Possible roles of PMP22 in the pathogenesis of CMT1a are discussed.