Objectives To determine the level of knowledge regarding iodine nutrition and its relationship with socio-economic status in the South African population. Design A cross-sectional population survey collecting questionnaire information on knowledge of iodine nutrition and sociodemographic variables in a multistage, stratified, cluster study sample, representative of the adult South African population. Setting Home visits and personal interviews in the language of the respondent. Subjects Data were collected from one adult in each of the selected 2164 households, and the participation rate was 98%. Results Only 15.4% of respondents correctly identified iodised salt as the primary dietary source of iodine, 16.2% knew the thyroid gland needs iodine for its functioning, and a mere 3.9% considered brain damage, and 0.8% considered cretinism, as the most important health consequence of iodine deficiency. Compared with respondents from high socio-economic households, respondents from low socio-economic households were considerably less informed about aspects of iodine nutrition covered in this study. Conclusions The knowledge level of iodine nutrition is low among South Africans, particularly among the low socio-economic groups. These data suggest that the international emphasis on brain damage resulting from iodine deficiency has not been conveyed successfully to the consumer level in this country.