The Institute of Nutrition of Central America and Panama (INCAP) longitudinal study of 1969 to 1977 was a community randomized trial in which 2 pairs of matched villages received either a protein-rich gruel ( atole) or a nonprotein, low-energy drink ( fresco). Both contained equal amounts of micronutrients by volume. I review the history and design of the study and impact on dietary intakes and physical growth. The design dates from the 1960s when protein was seen as the main dietary deficiency. During the 1970s, emphasis shifted to energy deficiency and this influenced early analyses. Energy from the 2 drinks during pregnancy was associated with improved birthweights and whether protein was also provided along with energy appeared to make no difference. These analyses, observational in nature, were possible because there was substantial overlap in energy intakes from the supplements during pregnancy across village types. In children, analyses initially focused on energy but eventually relied on the original experimental design. Exposure to the atole compared to fresco was associated with improved physical growth at 3 years of age but not from 3 to 7 years. Consumption of the fresco in the first 3 years of life was low such that there was little overlap in energy intakes from the supplements, not allowing for the type of analyses done for pregnancy. Diets in atole villages were greater from 15 to 36 months in protein, energy, and micronutrients, making attribution of impact on growth to a specific nutrient impossible. The atole improved linear growth, arm, and calf circumferences at 3 years but not skinfold thicknesses.