In the Darfur region of Sudan, 85.5% of 7134 subjects examined, mainly school children, had goitre and in 23.9% the goitre was large (stage II or III). The prevalence of goitre was high in prepubertal children of both sexes and in adult females. There was a higher frequency of large goitre in the rural than in the urban areas (40.9 and 11.4%, respectively). In Port Sudan on the Red Sea Coast the goitre rate was 13.5% among 7697 schoolchildren, but here visible goitre was extremely rare. Of the subjects from Darfur, 54.5% excreted less than 50 micrograms of iodine/g creatinine, while all except one subject in Port Sudan excreted more than that. The median urinary excretion of iodine was 45.3 micrograms/g in subjects from Darfur and 171.2 in those from Port Sudan, the mean value being significantly lower in the former than in the latter (p less than 0.001). The iodine content of all water samples was very low, and the contents of calcium and fluoride were not high. Other goitrogenic factors could not be excluded. However, iodine deficiency is the major cause of goitre in the Darfur region and a prophylactic programme is urgently needed.