Extract: The IQ's of Jamaican boys aged 6–10 were associated significantly with the presence or absence of severe malnutrition in infancy, with height at time of IQ testing, and with a measure of the boys' social background. A multiple correlation coefficient of 0.674 was obtained between IQ and the three factors. Social background contributed 0.294 of the variance, height 0.112, and severe malnutrition 0.049. The two extreme groups of boys, i.e., those malnourrished, small at follow-up, and with unfavorable social backgrounds and those not malnourished, tall at follow-up, and with favorable social backgrounds had average IQ's of 49.4 and 74.9, respectively (from Table 5). Only two of the boys in the most advantaged group had IQ scores that overlapped with the most disadvantaged group. Boys with severe malnutrition in infancy, but who are tall at follow-up and have a favorable social background have an average IQ 11 points higher than boys who did not experience severe malnutrition, but who are short at followup and have a unfavorable social background. The difference in IQ between boys who did and did not experience severe malnutrition in infancy varies under different conditions of height and social background when those are held constant for both groups. Under the most favorable conditions of being tall and having an advantageous social history the average IQ of the malnourished boys is only 2 points lower than those not malnourished. Under the most unfavorable conditions of short stature and a disadvantageous social background the IQ of the malnourished boys is 9 points lower than those not malnourished (Table 6 and Fig. 1). Speculation: The long range consequences of a severe episode of malnutrition in infancy can be understood only in the context of the human ecologic conditions including social, economic, and biologic factors which the child and his family have experienced during their lifetimes. Results of the present study suggest that an episode of severe malnutrition in infancy in the context of a lifetime of generally favorable experiences for child development does not appear to cause any intellectual impairment, but when severe malnutrition occurs in an ecology generally unfavorable for intellectual development, the early malnutrition has a clear relation to later intellectual impairment.