A single injection of iodized oil was administered intramuscularly in a dose of 0.2-2 ml (475 mg iodine per ml) to 383 children from the villages of Kas and Dibbis of the Darfur region in Western Sudan, where goitre is prevalent. After one year the prevalence of goitre had declined from 76% to 49.6%. Of the goitrous children, 81.1% showed either complete resolution of goitre or definite regression of its size. None of the nongoitrous children developed goitre and none of the existing goitres became larger. No cases of hyper- or hypothyroidism were observed. The full 270 children were followed up for four-and-a-half years after treatment and compared with untreated controls from the same area. The prevalence of goitre was significantly lower in the treated than in the untreated group. In treated children the mean urinary iodine excretion three-and-a-half years after treatment was 188.7 micrograms/g creatinine. This was significantly higher than in the untreated group (P less than 0.001). One year later the mean urinary excretion of iodine in the treated children had decreased to 87.1 micrograms/g creatinine. There was no difference in the mean serum values of T3 and T4 in the treated and untreated groups. The level of thyroid stimulating hormone was significantly lower in the treated [mean (S.D.) = 3.5 (0.9)] than in the untreated children [6.0 (3.5)]. It is concluded that a single injection of iodized oil is safe and effective in prevention of goitre development, reduces goitre size and maintains a sufficient iodine supply to prevent goitre for at least four-and-a-half years in children from goitre-endemic areas in Western Sudan.