question: INTRODUCTION Charcot-Marie-Tooth (CMT) disease, an untreatable hereditary polyneuropathy, may mimic chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), a treatable neuropathy. OBJECTIVE In this retrospective study, we analyzed the characteristics of CMT patients misdiagnosed as CIDP in 16 University Hospitals from 3 countries, compared these patients with a reference group of CIDP patients, and estimated the cost of misdiagnosis. RESULTS Amongst 1104 CIDP cases, we identified 35 CMT patients misdiagnosed as CIDP (3.2%). All were initially diagnosed with definite or probable CIDP (EFNS/PNS criteria), and mutations in the PMP22, MPZ and 10 other CMT genes were found in 34%, 31% and 35% of cases respectively. In comparison with a reference group of 35 CIDP patients, CMT patients were younger (median age at disease-onset: 39 vs 56 years), and more frequently had motor weakness at disease onset (80% vs 29%), hearing loss (14% vs 0%), normal brachial plexus imaging (70% vs 40%), lower cerebrospinal fluid protein content (median: 0.5 vs 0.8 g/L), and lower treatment response (20% vs 69%). Treatment cost in these 35 misdiagnosed patients was estimated at 4,6 Million € (M€), whereas the cost of CMT genetic analysis in 1104 patients was estimated at 2,7M€. CONCLUSION In this study, 35/1104 (3.2%) of patients initially diagnosed with CIDP had CMT. Importantly, the cost of treating these 35 misdiagnosed patients was significantly higher than the cost of performing CMT genetic analysis in 1104 patients (4,6M€ vs 2,7M€), suggesting that CMT genetic investigations should be more widely used before diagnosing CIDP.