Every child with failure to thrive has at least one organic medical disease: malnutrition. It is well documented that lead and other heavy metals are absorbed more readily in the presence of both malnutrition and iron deficiency anemia. Malnutrition and lead exposure tend to be found in the same population groups. Furthermore, lead poisoning is correlated with many of the identical intellectual and behavioral deficits demonstrated in children suffering from nonorganic failure to thrive. Because of these facts, whole blood lead levels were determined for 45 children with nonorganic failure to thrive and 45 age-, race-, and socioeconomically matched comparison subjects. Children with failure to thrive had a lead level of 22.67 +/- 10.29 (micrograms/dL (mean +/- SD); for control children, it was 14.33 +/- 5.42 (P less than .001). Children with failure to thrive were more frequently anemic (P less than .0001), a possible lead effect, and had higher free erythrocyte protoporphyrin levels. Children with failure to thrive were developmentally delayed on the Denver Developmental Screening Test (unblinded observation) with high failure rates in both language (P less than .001) and gross motor skills (P less than .02). Although failure on the Denver Developmental Screening Test within the failure to thrive group was not linearly correlated with lead level, any such effects may have been masked by the effects of malnutrition and failure to thrive per se. A number of authors have suggested that lead levels formerly thought to be inconsequential are clinically toxic.(ABSTRACT TRUNCATED AT 250 WORDS)