We investigated the duplication of the PMP-22 gene in 23 Japanese patients with Charcot-Marie-Tooth disease (CMT) by Southern blot and PCR analysis. To detect duplication of the PMP-22 gene region, PMP-22 cDNA and a polymorphic marker, VAW409R3, located in the region flanking the PMP-22 gene, were used as probes for Southern blot analysis. A marker, 6G1, also located in the PMP-22 flanking region, was amplified using the quantitative PCR method. The signal intensity of the 2.8-kb and 2.7-kb bands of MspI digests probed with VAW409R3 was different in patients with duplication. The ratio of these two bands, measured by densitometry, ranged from 1.75 to 2.13 in the patients with duplication and from 1.01 to 1.15 in those without duplication and in normal controls. The signal ratio of PMP-22 to the reference marker SF85 of BamHI digests ranged from 1.15 to 1.33 in the patients with duplication and from 0.96 to 1.04 in those without duplication, when compared with normal controls assigned a value of 1.0. The ranges of the intensity in these two groups were narrow, but did not overlap. The signal ratio of the PCR products of 6G1 to reference marker D1S80 on quantitative PCR analysis was also measured. The ratio ranged between 1.67 and 2.23 with duplication and between 1.29 and 1.74 without duplication according to the results of Southern blot analysis. However, some patients exhibited overlapping signal intensity in the duplication and non-duplication ranges. Thus, these three methods each has its own advantages and disadvantages in regard to detecting duplication.(ABSTRACT TRUNCATED AT 250 WORDS)