A 6-year-old boy lost his normally-developed language ability within 2 months after bilateral sudden peripheral deafness. The boy became non-communicative with others, restless with frequent meaningless bursts of shouts (communication skills equivalent to 9-11 months of language development: a rapid breakdown of language). Since conservative methods were ineffective, cochlear implantation was performed. A surprising success was observed: he regained the language retracing the normal developmental stages and caught up with his contemporaries in 2 years. Behavioral improvement paralleled his language development. This case (1) provides insights into the brain function with respect to language acquisition, in relation to the plasticity during the 'critical period' of language learning, (2) reveals the close relationship between language development and behavior, and (3) suggests the predominance of auditory stimulation in learning language.