Children with congenital severe combined immunodeficiency disease (SCID) lack an immune system and require continuous reverse isolation to protect them from exogenously transmitted infection. In our setting, protected environments consist of laminar flow rooms in which everything is sterilized (food, clothing, toys and so on). Staff and family wear complete surgical garb, preventing skin contact, observation of mouth movement and olfaction of typical human odours. Case studies of language development in children reared from infancy in protected environments have cited instances of mental retardation1 or language delay with some improvements after intervention, although the degree of language exposure is not well documented2-5. We report here the case of a male patient, successfully treated for SCID, who lived in reverse isolation from 9 months to 4 yr 4 months of age and who has demonstrated nearly complete recovery of language usage. Our study illustrates the resilience of human intellectual function following a condition of early severe deprivation. © 1983 Nature Publishing Group.