Charcot-Marie-Tooth disease type 1A (CMT1A) is a common hereditary demyelinating neuropathy caused by a duplication of the gene for the myelin protein PMP22, resulting in overexpression of PMP22 in young patients. Although genetically well defined, the pathogenesis of the hereditary demyelinating neuropathy CMT1A is still unclear. Homology of PMP22 cDNA to the growth arrest-specific gene gas3 and experiments in vitro showing decreased proliferation in PMP22-overexpressing Schwann cells suggest a role of PMP22 in Schwann cell differentiation. Furthermore, overexpression of PMP22 in fibroblasts induces programmed cell death. In this report we applied morphometrical methods using electron micrographs and immunohistochemistry to further characterise Schwann cells in CMT1A nerve biopsy samples from CMT1A patients. We show that the total number of PMP22-expressing Schwann cells, i.e. Schwann cells that are in a 1:1 relationship with axons, was not reduced in sural nerve biopsy samples from six young CMT1A patients. We excluded non-specific secondary Schwann cell proliferation. Thus, in young CMT1A patients with increased PMP22 overexpression there seems to be no evidence for altered initial Schwann cell proliferation in achieving a 1:1 relationship to axons prior to the process of de- and remyelination. Further, using electron microscopy we found no evidence for apoptosis of Schwann cells in CMT1A. However, we provide additional support for an abnormal Schwann cell phenotype in CMT1A by showing the expression of neural cell adhesion molecule immunoreactivity in onion bulbs. Thus, the role of PMP22 in cell growth and differentiation does not lead to an altered number of myelinating Schwann cells but to altered Schwann cell differentiation in CMT1A.