A nationwide sample survey was conducted in Chad to establish the prevalence of iodine deficiency disorders (IDD). The country was stratified into the Sahel zone and the Sudan zone, the latter including the city of N'Djamena. The analysis followed a stratification whereby the city of N'Djamena was also separately analysed. A total of 1171 people between 10 and 20 years of age were included in the survey. The overall weighted prevalence of goitre, evaluated by a clinical examination, was 63%. In the Sudan zone the prevalence was 70%, in the Sahel zone 64% and for the capital, N'Djamena, 25%. There were significant differences in the frequency of goitre between the three zones. Females-as established in surveys from other areas and countries-had goitre significantly more often. The prevalence of cretinism varied between 0 and 1.2%, and was highest in the Sahel zone. Amongst the sampled population of the Sudan zone, 33% had less than 20 micrograms/l of urinary iodine, indicating severe iodine deficiency. In the Sahel zone and in N'Djamena the figures reached 8 and 1%. There were high levels of thiocyanate anions in urine reaching medians between 21 and 27 mg/l in the geographical stratas. This probably decreases the bio-availability of iodine. These data show that there is a high endemicity of goitre in Chad and provide a basis for interventions as part of a national action plan against IDD, which will be adapted to the social, cultural and economic situation of the country and to available health services.