This paper investigates the relationship between intake of two types of supplements, caloric (fresco) and protein-caloric (atole), on the urinary U/C ratio in children from four rural Guatemalan villages. Atole was distributed in two of the villages while in the other two fresco was provided. Daily amounts of individual ingestion of supplement were recorded for lactating mothers as well as for breast-fed and weaned children up to 84 months of age. The urinaryU/C ratio of the children was determined in a casual morning urine sample.Positive associations were observed between supplement ingestion in lactating mothers and the U/C ratio of breast-fed children in atole (r = 0.288, n = 77, p < 0.01) as well as in fresco (r = 0.148, n = 103, p < 0.10) villages. In contrast, the relationship between ingestion of the supplements in breastfed and weaned children and the U/C ratio differed depending upon the type of supplement given. Thus, for the caloric supplement, fresco, the relationship was negative (breast-fed children: r = 0.150, n = 103, p <0.10; weaned children: r =0.145, n = 834, p < 0.001) while for the protein-calorie supplement, atole, the relationship was positive (breast-fed children: r = 0.645, n = 77, p < 0.001; weaned children: r = 0.261, n = 724, p < 0.001).The positive relationship observed between protein-calorie supplementation in lactating mothers and the U/C ratio in their children suggest that protein ingestion of the lactating women increases protein output in breast milk. The findings with respect to caloric intake, deserve more extensive comment. It was concluded that these findings bring support to the hypothesis that calories are the main limiting factor in the diet of this population. Thus, giving calories to the lactating mother may have resulted in enough protein spared to increase the breast milk protein supplied to the suckling child and consequently to increase his U/C ratio. Similarly, the reduction observed in the U/C ratio of the child upon supplementing him with calories may indicate that the calories given spared protein ingested at the home from being used to provide energy.In spite of the fact that the results, the first from free-living communities reported in the literature, suggest that caloric supplementation to lactating mothers and children led to a betterment of their nutritional status, it should be stressed that decision to implement calorie supplementation alone in other populations should deserve careful consideration of the main limiting dietary factors to avoid the possibility of increasing the risk of protein malnutrition. © 1981 Oxford University Press.