A study of some environmental, behavioral and social factors in the production of lead poisoning in children in an urban community has been reported. Most significant among the environmental data presented were the magnitude of the exposure to lead from repetitive ingestion of small quantities of leaded-paint flakes, and the role of the seasonal factor (summer) in the precipitation of acute lead encephalopathy. The predominance of these two factors may explain the absence of any significant correlation between severity of disease and increments (beyond a minimum of 3 months) in the probable duration of exposure. It was concluded that a child without symptoms but having increased absorption of lead, recognized during the summer months, may progress to severe encephalopathy within a few weeks if the sources of lead are not promptly identified and eliminated. The present studies demonstrate that exposure to lead may be intense in children over 1 year of age living in dilapidated dwellings in which flaking leaded paint is readily accessible. The amounts of lead found in the feces of the poisoned children in the present study exceeded that found by others in the feces of exposed industrial workers. This suggests that the higher incidence of lead encephalopathy among children as compared with adults may, in part, result from their relatively greater exposure. The importance of continued environmental exposure to lead in increasing the incidence of severe permanent damage to the brain among survivors of an initial attack of acute lead encephalopathy was demonstrated. The correlation between the occurrence of such sequellae and re-exposure to lead in patients recovering from mild acute encephalopathy was statistically highly significant. It was concluded that removal of lead from the child's environment is the only adequate protective measure in such cases. The high proportion of children aged 12 to 35 months with average intellectual capacity found among cases of lead intoxication, and the high incidence of unsuspected cases found among 12- to 35- month-old housemates of the index cases demonstrate the importance of the environmental aspect of the problem in urban slum areas. This high incidence of "secondary cases" emphasizes the physician's obligation to examine carefully and promptly all environmental contacts under 3 years of age whenever an index case is found. The developmental factors in the child and the social situations in the home which may intensify the ingestion of lead-containing materials were discussed. While the responsibility of parents to protect their children from environmental hazards is not denied, no mother can reasonably be expected to prevent the repetitive ingestion of a few paint chips when these are readily accessible. As lead is widely used as a paint drier, continued good maintenance of painted surfaces would appear to be more pertinent to the long-range prevention of childhood plumbism than specific limitations on the content of nonpigment lead in fresh paint. The various environmental data all point to the conclusion that, where housing has been permitted to deteriorate, exposure to lead may be of such intensity as to outweigh such individual variables as mental retardation and emotional maladjustments in the child. Such intense exposure is a preventable hazard to normal small children, and, as such, constitutes a public health problem which may be more extensive than has heretofore been thought to exist. Pending the wider availability of better housing, preventive measures will have to be adapted to individual home situations and to the available facilities within a given community. Basic to any preventive program are facilities for the prompt identification and removal of environmental sources of lead, as has been emphasized by others, Equally important is a comprehensive social investigation of the home in order to evaluate the circumstances under which the child obtained toxic quantities of lead and to determine whether the identified sources of lead can be adequately removed, or whether a change of dwelling is required to prevent further dangerous exposure to lead.