Cognitive and emotional aspects of development in four infants reared in a reverse isolation environment because of congenital severe combined immunodeficiency disease were studied by psychological test performance and formal observation. The children were studied while they were inpatients and following their discharge after successful medical treatment was accomplished. Treatment time in reverse isolation varied from 10 to 52 months. Deficient self-generated activity, including motor and motor-based cognitive skills such as expressive language, were observed in two of the four children. Deficits were also observed to be at least moderately reversible either upon discharge or in relation to an inpatient intervention program. Case material is discussed with reference to severe disruption of oral feeding experience, quality of parental involvement, and sensory isolation inherent in the environment.