We describe a nutritional intervention by the Rockefeller Foundation's International Health Division in Spain after the Spanish Civil War, delineating the relationships between the technicians sent by the Rockefeller Foundation and the Spanish health authorities. We analyze reports of the nutritional situation in Spain in the early 1940s and the design and outcomes of a nutrition survey conducted in a district of Madrid by American and Spanish nutritionists. This nutritional survey, which was based on food intake interviews and was complemented with anthropometric measurements, clinical examinations, and blood tests, found several symptoms and signs of malnutrition. The Rockefeller Foundation's nutritional research was an important historical precedent for later studies made in emergency situations or armed conflicts. Similar surveys have been carried out in the last several decades by distinguished academic departments of public health and epidemiology and by humanitarian aid agencies.