We analyzed a 1.5-Mb duplication of the p11.2-12 region of chromosome 17, including the PMP-22 gene (CMT1A duplication), seven families with Charcot-Marie-Tooth disease type I (CMT I) and six sporadic patients with suspected CMT I by Southern blot analysis. In order to detect the CMT 1A duplication, probe pVAW409R3a, probe PMP-22 cDNA and reference probe SF85 were used for Southern hybridization. In six out of seven families with CMT I, CMT1A duplication was identified. One of six sporadic CMT patients had CMT1A duplication. The probe pVAW4O9R3a was more informative than PMP-22 cDNA and SF85 for detecting CMT1A duplication. In pathological study of biopsied sural nerve, thickened myelin sheath was observed in some myelinated fibers in patients with CMT1A duplication.