Endemic goiter is a health problem in many areas of the world; in some areas the disease is so severe that cretinism and other defects are found. In many areas geographic, economic, and other factors prevent the use of iodized salt as a preventive measure. Field studies were begun in 1966 to determine the feasibility and effectiveness of parenteral administration of iodized oil in goiter prevention. Studies were carried out in Ecuador and Peru. In Ecuador 2 villages were chosen in which the prevalence of goiter was about 60%; in Peru 3 villages were chosen where incidence was about 50%. Prevalence of goiter decreased for 20 months during the study but then began to rise again with the maximum reduction seen up to age 18 and minimal reduction after 40 years of age. The control groups in the study experienced only slight decreases in rate of incidence. Cretinism has not yet appeared among the progeny of the population injected with iodized oil but several instances have appeared in control groups. The use of iodized oil as a public health procedure for the prevention of endemic goiter and its associated defects is an acceptable measure in regions where salt iodization cannot be done.