A Japanese man with a negative family history of paramyotonia congenita (PMC) was evaluated for symptoms of cold-induced weakness and stiffness. Exercise testing revealed findings characteristic of PMC, and a genetic analysis was therefore performed. A well-known sodium channel mutation for PMC (T1313M) was identified in the patient, but was absent in his biological parents. These data demonstrate the occurrence of a de novo mutation, suggesting that evaluation for PMC should be performed in patients with typical symptoms even if the family history is negative.