Thirteen very poor Peruvian families have been followed for a number of years. Fifteen of their children were admitted to the hospital severely malnourished, with a mean height quotient (height age X 100/chronologic age) of 45, at a mean age of 10.6 months. At 3 years of age, mean height quotient was 60.9; at 7 years of age it was 68.9. Forty so-called healthy siblings had height quotients of 62.8 at 1 year, 65.3 at 3 years, and 73.2 at 7 years of age. From 3 to 7 years of age differences were no longer significant. Eighteen siblings were given an optimal diet in a protected environment from shortly after birth until 18–27 months of age. At 18 months of age their mean height quotient was 82.5. Those who remained in the protected environment had a quotient of 85.0 9 months later; those who returned home had a mean height quotient of 64.9 1 year later, no longer significantly different from the “malnourished” and “healthy” siblings. There was no difference in head circumference, at the same height, between the recovered malnourished and those whose nutrition was ideal during the first 18–27 months of life. The head circumference, at the same height, was less than the U.S. 50th percentile and more than the 3rd percentile, suggesting a different genetic constitution.SpeculationAdverse environmental influences affecting nutrition over the entire growth period are probably as important as inheritance in determining eventual stature of children. Catch-up growth, both in height and in head size, can go on for many years after a period of severe malnutrition. Head size, and presumably brain mass, may not be selectively affected by severe malnutrition in early life and may remain a function of body mass