Background: Mutations in the peripheral myelin protein 22 (PMP- 22) gene are the most common cause of Charcot-Marie-Tooth neuropathy and may rarely occur in combination with other neurogenetic diseases. Objective: To characterize 3 families having a mutation in PMP-22 in addition to another neurogenetic disease mutation. Design: Clinical, electrophysiologic, and genetic evaluations were made of 3 families with more than 1 genetic neuromuscular disease. Setting and Patients: Family members were evaluated in neurogenetic and muscular dystrophy clinics in a university medical center setting. Results: Three unusual families were found: (1) 2 young brothers each having a PMP- 22 duplication and a missense mutation in the GJB1 (Connexin- 32) gene; (2) a 32- year- old woman having a PMP- 22 duplication and a 1000- fold CTG repeat expansion in the DMPK gene (DM1 myotonic dystrophy); and (3) a 39- year- old man with a PMP- 22 deletion and a missense mutation in the ABCD1 gene (adrenomyeloneuro-pathy). The mutations were “ additive,“ causing a more severe phenotype than expected with each individual disease and coinciding with the important impact of each gene on peripheral nerve function. Conclusions: Individuals having 2 separate mutations in neuromuscular disease-related genes may develop unusually severe phenotypes. Neurologists should be alert to this possibility. Background: Mutations in the peripheral myelin protein 22 (PMP-22) gene are the most common cause of Charcot-Marie-Tooth neuropathy and may occur occur in combination with other neurogenetic diseases. Objective: To characterize 3 families having a mutation in PMP- Design and Clinical Evaluations were made of 3 families with more than 1 genetic neuromuscular disease. Setting and Patients: Family members were evaluated as neurogenetic and muscular dystrophy clinics in a university medical center setting. Three unusual families were found: (1) 2 young brothers each having a PMP-22 duplication and a missense mutation in the GJB1 (Connexin-32) gene; (2) a 32-year- old woman having a PMP - 22 duplication and a 1000-fold CTG repeat expansion in the DMPK gene (DM1 myotonic dystrophy); and (3) a 39-year- old man with a PMP- 22 deletion and a missense mutation in the ABCD1 gene (adrenomyeloneuro- pathy). The mutations were ”additive, " causing a more severe phenotype than expected with each individual disease and coinciding with the important impact of each gene on peripheral nerve function. Conclusions: Individuals having 2 separate mutations in neuromuscular disease-related genes may develop unusually severe phenotypes. Neurologists should be alert to this possibility.