Résumé : It has been generally recognised that sick children have an increased risk on malnutrition. An activated immune system requires more nutrients while illness is often associated with a lower absorption and decreased consumption because of lack of appetite. When these increased needs are not balanced with an adequate availability of nutrients, the sick child is at higher risk of developing malnutrition.Médecins Sans Frontières investigated the question whether this process is mitigated by simple short term nutritional supplementation given to sick children alongside medical treatment. Three Randomised Controlled Trials (RCT’s) were conducted. The first, in Democratic Republic of Congo, was a pilot; 180 children with malaria were randomised in 2 arms: 1 group receiving 2 weeks of ready to use therapeutic food (RUTF) and a control group. The children were followed for a period of 4 weeks. Children in the RUTF group showed a higher weight gain in the first 14 days compared to the control group, at day 28 the weight gain in both groups was similar.Thereafter, 2 RCT’s were implemented in Uganda and Nigeria using a similar methodology. Children with malaria, lower respiratory tract infection or diarrhoea (sample size of 2202) were randomised in three groups: supplemented with 2 weeks of RUTF, supplemented with 2 weeks of micronutrient powder (MNP), and not receiving supplementation after each disease episode. The incidence of malnutrition was compared after an observation period of 6 months. The trial in Uganda showed a reduction in malnutrition in the RUITF group with 31%, while in Nigeria, there was no significant reduction in the RUTF group. The MNP group did not show reduction in malnutrition in any site. In the group of moderate malnourished children the RUTF and MNP supplementations were not effective in preventing deterioration to severe malnutrition. However, when the studies were combined the RUTF group showed a lower mortality compared to the MNP group.Multi-variate analysis did not show a reduction of incidence of malnutrition in the supplementation groups. A strong association with morbidity was found. A higher frequency of diarrhoea was associated with an increased incidence of malnutrition. The association with malaria episodes was mixed; it was associated with a higher incidence of malnutrition in Kaabong, but in Goronyo a higher frequency of malaria decreased the incidence of malnutrition. In addition, a more frequent monitoring of the children and treatment of their illnesses was associated with a decreased incidence of malnutrition.The difference in effectiveness of supplementation between the sites can be explained by differences in food security and level of morbidity. It is argued that the fragile food security in Kaabong limits the supply of nutrients, and therefore supplementation with RUTF was effective. In Goronyo the high frequency of morbidity limits convalescence and therefore supplementation was not effective.It is likely that malnutrition is more effectively prevented when several interventions are combined like water and sanitation to prevent diarrhoea, malaria chemoprophylaxis and preventative and curative health and nutrition interventions.This dissertation will present the background, the methods of the trials and the results, followed by a discussion on the implications for programming and research.