In developing countries, more than 12 million children die each year from the combined effects of malnutrition and infection. Malnourished children have impaired cellular immunity and are particularly sensitive to opportunistic infections. However, immune recovery has rarely been investigated during nutritional rehabilitation. Indeed, mortality remains high during renutrition, and relapses are frequent. We established a center in Cochabamba, Bolivia, specifically to save these children by treating both clinical and nutritional problems and restoring immune function. The CRIN (center for immuno-nutritional recovery) admits children with severe malnutrition from the Cochabamba suburban area. They are from low income families, in crowded living conditions with poor sanitation and are weaned early. Nutritional diagnosis was based on weight-for-height, arm to head circumference ratio and clinical examination for edema, loss of subcutaneous tissue and diminished muscle mass. The children were examined daily, and first treated for respiratory and intestinal infections. Sociological and psychological aspects were also included in our holistic approach to treating severe malnutrition. Children received a four-stage diet lasting 2 months. During the initial phase (1 week) they were given an oil-sugar-milk based diet, with half lactose concentration, seven times a day. This supplied 1.5 to 2.5 g of protein and 120 to 150 kcal/kg of body weight, according to the PEM pattern. Protein and energy intake was then slowly increased during the transition phase (1 week). During the next, 'calorific-protein bombing' phase (6 weeks) 5 g of protein and 200 kcal/kg of body weight were given daily, such that there was sufficient energy for protein accumulation. During the last, discharge phase (1 week), the protein and energy contents were slowly decreased. Weight, height, arm and head circumferences, and triceps skin-fold thickness were measured weekly by standardized methods. Thymus size was assessed weekly by mediastinal ultrasound scanning with a portable scanner (ALOKA SSD-210 DXII, Tokyo) using a 5 MHz linear pediatric probe. Lymphocyte subpopulations in peripheral blood were investigated monthly using monoclonal antibodies. Compared to controls, the malnourished group had severe involution of the thymus, a significantly higher proportion of circulating immature T lymphocytes and a lower proportion of mature T lymphocytes. The two month longitudinal study showed that normal anthropometric values (90% NCHS weight for height) were recovered after one month of rehabilitation. However, immune recovery (thymic area of 350 nm2) required two months. This may explain the frequent relapses among malnourished children discharged after one month on the basis of 'apparent nutritional health'. Such children may remain immunodepressed, and should therefore be considered as high risk children. To test an immunostimulatory treatment, we designed a historical cohort study of malnourished children who received 2 mg of zinc per day. The children were matched for age, sex, anthropometric criteria and nutritional status with malnourished control children (treated previously with zinc). Anthropometric recovery was obtained in both groups in one month. Children receiving zinc attained immunological recovery within one month, whereas children not receiving zinc took two months. Thus zinc hastened immunological recovery concomitant with nutritional recovery such that the duration of hospitalization could be halved: after one month of this immuno-nutritional treatment, malnourished children appear to be sufficiently healthy to face their pathogenic home environment.