We investigated the presence of duplication in chromosome 17p11.2 in 4 individuals with sporadic Charcot-Marie-Tooth disease (CMT 1) and 1 isolated case where a definite differential diagnosis between CMT 1 and Dejerine-Sottas disease was not achieved. The 5 affected cases and their parents and relatives were submitted to a complete clinical, neurophysiologic and genetic evaluation. A sural nerve biopsy was performed in all the isolated patients. Paternity was tested and confirmed. The presence of DNA duplication was detected in all the sporadic cases and was absent in all parents and relatives, thus confirming that a de novo dominant mutation is commonly present also in patients without a familial history and that there is a practical relevance of the genetic study in distinguishing isolated cases of CMT 1 from other forms of hereditary motor and sensory neuropathies or demyelinating neuropathies.