The purpose of this study was to assess the validity of insulin-like growth factor (IGFI) determination as an index of nutritional status and growth in Equadorian schoolboys. Plasma IGFI was measured in 144 healthy boys, 9 years old, who were classified by their social class in four groups: 1 (n = 29); 2 (n = 49); 3 (n = 28); 4 (n = 8). Children in groups 1 and 2, of the lower socio-economic class, had significantly reduced caloric and protein intakes compared to children in groups 3 and 4 (P less than 0.01). Mean heights were significantly lower in groups 1 and 2 (P less than 0.001 and less than 0.05, respectively). The mean plasma IGFI in group 1 was 561 +/- 64 mU/ml and significantly lower than that in groups 2-4 (872 +/- 75, 986 +/- 94, and 1238 +/- 190 mU/ml in groups 2, 3 and 4, respectively) (P less than 0.01). The effect of caloric and nitrogen supplementation was studied in 36 children taken from groups 1 and 2 combined, and divided into three subgroups; they were given (1) an animal protein supplement of 18 g/day with 860 kcal/day (n = 12), (2) a vegetable protein supplement of 17.8 g/day based on a traditional Andean food with 862 kcal/day (n = 12), and (3) a placebo (n = 12). The IGFI levels increased significantly after 7 and 14 days in both subgroups 1 and 2. These results demonstrate that plasma IGFI reflects the nutritional status and responds to a short-term diet supplementation. It may provide a reliable means of assessing the effect of nutritional programs intended to improve the growth of children in underdeveloped countries.