Acquired epileptic aphasia (AEA, or Landau–Kleffner syndrome) is a unique condition in which children can lose oral language (OL) comprehension and expression for a prolonged period. These children can benefit from visual forms of language, mainly sign language (SL), but the quality of SL has never been analyzed. The case is reported here of a boy with AEA who lost speech comprehension and expression from 3 years 6 months to 7 years and was educated in SL from the age of 6 years. His SL was evaluated at the age of 13 years and 6 months and compared with a control child with congenital sensorineural deafness. It was found that: (1) our patient achieved the same proficiency in SL as the control child with deafness; (2) SL learning did not compete with, but perhaps even hastened, the recovery of OL. Intact ability to learn a new linguistic code such as SL suggests that higher-order language areas were preserved and received input from a separate visual route, as shown by neuropsychological and functional imaging research in deaf and hearing signers.