A 1.5 Mb duplication within 17p11.2 is the major mutation causing both autosomal dominant and sporadic Charcot-Marie-Tooth disease type 1A (CMT1A). An independent origin for the mutation in each family has been postulated. The proposed genetic mechanism causing the CMT1A duplication is unequal nonsister chromatid exchange at meiosis (unequal crossing-over). We studied the parental origin of the duplication in nine genetically sporadic CMT1A patients and demonstrated that in all cases the mutation was the product of an unequal nonsister chromatid exchange during spermatogenesis. The fact that only paternal de novo duplications were observed in the sporadic CMT1A patients suggests that male specific factors may be operating during spermatogenesis that either help forming the duplication and/or stabilize the duplicated chromosome.